Similar to visual acuity, contrast sensitivity is an important parameter to characterize visual capacity. Measurements of contrast sensitivity can be used to gain insights into the capability of the visual system to recognize objects varying in size at low contrast. This is extremely relevant for practical applications because daily routines often require objects to be perceived in the presence of poor light conditions and poor contrast, e.g., rain, fog, and darkness. The ability to see contrast can be affected adversely by eye disease or inadequate eyesight correction. Contrast sensitivity is an important parameter for evaluating visual performance. Moreover, in measurement and correction of higher order aberrations, contrast sensitivity is often used to assess improvement in eyesight. To be suitable for this purpose, a test has to be rapid and exact.
Various test patterns to determine contrast sensitivity are in use (Fig. 1). One option is the use of sharply delimited optotypes, for example, letters or Landolt rings. The use of letters is advantageous because the level of chance response is low. The subject is presented with 1 of 26 different letters, although usually only 10 letters are actually used. Reading letters aloud is a task that the subjects commonly face.1 However, it is not detection threshold which is being measured but rather identification threshold.2 Moreover, sharply delimited optotypes include a broad spatial frequency spectrum. Examples of contrast sensitivity tests using letters include the Pelli-Robson chart,3 the Bailey-Lovie chart,4 and the Mars Letter Contrast Sensitivity Test.5,6
A second option for measuring contrast sensitivity is the use of patterns of stripes. In many cases, light and dark areas are not sharply delimited; rather, luminance presents a sinusoidal distribution. In contrast to letters, gratings of this type allow detection threshold to be determined.2 These can be used to measure various spatial frequency channels of the visual system separately.7 A disadvantage of grating patterns is that the subjects are not familiar with detection. In addition, level of chance is higher.8 First, investigations with sine-wave gratings to measure human contrast sensitivity function (CSF) were done by using oscilloscope screens to produce the patterns.9–11 Several years later, Arden and Jacobson12 published photographic plates containing a range of spatial frequencies (increasing from left to right) and different contrasts (decreasing from the bottom up). Today, for clinical measurements, gratings are presented on computer monitors or charts. Currently known tests that use sinusoidal gratings as test patterns are the Vistech chart13 and its descendant, the Functional Acuity Contrast Test (FACT).7 Another example is the CSV-1000E (VectorVision, Greenville, Ohio).
In several studies, Simas et al.14,15 and Santos et al.16 used rotationally symmetrical targets that feature concentric circular light and dark areas with brightness varying in a radial sinusoidal way. The stimuli were modulated by spherical Bessel functions. Using these patterns, contrast sensitivity as a function of age were similar to those obtained as with sinusoidal gratings.16 Similar patterns (except for the absence of a modulation by Bessel functions) are implemented in a contrast sensitivity test system that is commercially available as Holladay Automated Contrast Sensitivity System (M&S Technologies Inc, Skokie, Illinois).
Aside from classification by the employed test pattern, contrast sensitivity tests can also be subdivided according to how they are presented to the subject. Test patterns can be presented either on charts or on a monitor controlled by a computer. Most of the contrast tests using charts have relatively broad increments. Step sizes of the Vistech chart are irregular (between 0.11 and 0.37 log units, average about 0.25 log units).13 The MARS test has narrow increments of 0.04 log units, but there is only one letter per step.5 The Pelli-Robson chart contains groups of three letters with increments of 0.15 log units from one group to the next.3 Subtle but subjectively noticeable changes in contrast sensitivity (e.g., from incipient cataract), however, are difficult to capture by these approaches. Computer-controlled presentation on monitors allows for finer step increments and intelligent strategies for detecting the contrast sensitivity threshold, e.g., by staircase methods or methods of adjustment.
The Siemens star has not previously been used as a test pattern for measuring contrast sensitivity. It was developed in the 1930s by Siemens and Halske AG, a predecessor of today's Siemens Company, as part of an optical test method for objective lenses for cameras. The star consists of black and white stripes in an alternating radial arrangement (Fig. 2A). The Siemens star is mainly used as a test marker for projection in optical systems or serves for fine adjustment and optimization of these systems.17 The black and white fields are sharply delimited from each other. Important parameters in assessments of system imaging are the imaging of the black and white edges and the sharpness of the central area.
In applications directly for the eye, the Siemens star has, to date, only been used in several autorefractometers as a test marker for determining objective refractive values.18
The modified version of the Siemens star comprises no sharp transitions between light and dark but rather a sinusoidal luminance distribution (Fig. 2B). Simas et al. and Santos et al. used similar patterns in testing subjects for the contrast at which the pattern as a whole is recognized. This was tested for various angular frequencies (=number of dark stripes) and contrast threshold was plotted as a function of angular frequency.19,20
However, use of the Siemens star to determine contrast sensitivity as a function of spatial frequency is novel and was presented for the first time at the 2008 ARVO conference.21
Principle of Measurement/Software
The modified Siemens star was used in this study to determine contrast sensitivity for different spatial frequencies. A star contains a continuum of all spatial frequencies within a range that is given by the angular frequency of the star (number of light and dark fields per cycle). Near the center, the stripes are very close together, for the high spatial frequencies. The distances in peripheral areas are larger and spatial frequencies are lower as a result. In the center of the Siemens star, spatial frequencies are below resolution and the pattern is perceived as uniformly gray. In the periphery, light and dark spokes can be distinguished because of perceivable spatial frequencies. The pattern includes a black ring (Fig. 3) intended to exactly mark the transition between the areas that can be resolved and those appearing gray in the center.
The newly developed test is called the Star-Ring Test. It generates the Siemens stars using software programmed in Flash (Adobe Systems) and displays them on an LCD monitor. The monitor resolution causes interfering artifacts in the center of the star (Fig. 4), which need to be covered by a round gray mask (Fig. 3). The luminance of this mask, similar to the luminance of the surroundings, corresponds to the mean luminance of the pattern.
To display the contrasts on the monitor correctly, a luminance calibration must be performed before the test. This is necessary as the correlation between the gray values generated and the luminance is not linear and varies across monitors.22 The approximate correlation is described by the following equation of an exponential function:
where L is the luminance, and ν corresponds to the gray value (0 to 255). The exponent is the Greek letter “gamma” and, accordingly, the luminance calibration is also called “gamma correction.” To determine the monitor-specific gamma value, an empirical measurement is made in the Star-Ring Test using the method described by Ward.23 Two fields are displayed side by side on the monitor, the left field containing alternating horizontal lines of 10 and 90% of the maximal luminance. The luminance of the field as a whole is equal to 50% of the maximum luminance. The field on the right has a uniform gray value that is varied during measurement so that the field attains the same brightness as the field on the left in the observer's view. The program uses this adjusted gray value ν to calculate the gamma value γ according to the following equation:
Equation (Uncited)Image Tools
The gamma value is then stored in the program and included in the calculation during generation of the test patterns.
Equation (Uncited)Image Tools
The Star-Ring Test enables two different measuring methods. First, spatial frequency can be defined and the corresponding contrast sensitivity determined (contrast adjustment mode). Second, a certain contrast value can be set and subsequently the marginal perceivable spatial frequency determined (ring adjustment mode). These two modes are illustrated in more detail in the following sections.
Contrast Adjustment Mode
The spatial frequency is set before the measurement. Thus the corresponding size of the black ring is defined for each presentation of the pattern. The ring diameter varies between 1/4 and 1/2 of the diameter of the Siemens star. This corresponds to an extension in the visual field of 0.52° to 1.05°, based on a test distance of 6 m. Hence, angular frequency amounts to at least 5 cycles/360° for a spatial frequency of 3 cycles per degree (cpd) and maximal 78 cycles/360° for a spatial frequency of 24 cpd. The subject has to press a button corresponding to the method of adjustment to increase or decrease the contrast of the Siemens star in increments of 0.10 log units as far as the ring marks the resolution threshold. Then the adjustment is noted and corresponds to the contrast threshold that is stored in the program. Then a star with a different randomly selected angular frequency is displayed. The contrast is 0.2 to 0.4 log units (continuously, also randomized) higher than the preceding one. The black ring again marks the selected spatial frequency and the subject has to adjust contrast. After 5 or 10 presentations of the pattern, the display shows the mean contrast thresholds adjusted by the subject (Fig. 5). To determine the profile of the CSF, this process is repeated for different spatial frequencies.
Ring Adjustment Mode
The contrast of the star has to be set before measurement. According to the method of adjustment, the subject has to press a button to adjust the size of the ring almost continuously (in steps of 0.01 log units). Once the ring marks the border between the resolvable and the non-resolvable area of the pattern, the subject confirms the entry. The software then uses the ring size to calculate the corresponding spatial frequency and stores the calculated value. In the next presentation, a star with a different angular frequency, but with the same contrast, is shown.
Subjects and Procedures
The study was divided into three experiments. Different groups of subjects participated in each experiment. A commercially available standard laptop with a graphics output was used to control the Star-Ring Test. The test patterns were displayed on a 15-inch TFT monitor (MultiVisus, made by bon Optic); the contrast was adjusted by subjects using an attached PC keyboard. The mean luminance of the Siemens stars was 179.2 cd/m2. The test room was moderately illuminated at horizontal illumination level of 123.1 lX.
The research procedures conformed to the tenets of the Declaration of Helsinki. Participation was voluntary, and the individual data obtained were kept confidential. The subjects were given an oral presentation of the purpose of the study and were advised that they could withdraw from the tests at any time.
In this experiment, we investigated whether measuring results vary when the Siemens star is stationary or rotates at a certain speed. The most comfortable rotational speed for the majority of the subjects was determined in preliminary experiments and was 20° per second in contrast adjustment and 10° per second in ring adjustment. In addition, we determined whether 5 or 10 presentations per trial are necessary. Ten subjects between 24 and 35 years of age participated in the experiment. The average age was 28.1 years with a standard deviation of ±3.5 years. The subjects were optometry students or optometrists; hence, they were familiar with psychophysical testing. Six of the subjects were female and four were male. Well-corrected visual acuity (≥0.8) was a prerequisite for participation in this experiment.
As a rule, subjects' right eyes were tested, their left eyes occluded. Contrast threshold was determined for four different spatial frequencies (3, 6, 12, and 24 cycles per degree), and the subjects adjusted the maximal resolvable spatial frequency at five different contrast values (1.25, 2.5, 5, 10, and 20%). The sequence varied randomly from one subject to the other. Stationary and rotating sinusoidal Siemens stars with a total extension in the visual field of 2.1° were used as test patterns. Rotation speed was 20° per second in the contrast adjustment mode and 10° per second in the ring adjustment mode. The test distance was 6 m.
The mean logarithmic contrast values and the standard deviations of 10 presentations were displayed as the result of each contrast adjustment trial. The definition according to Michelson (KM = (Lmax − Lmin)/(Lmax + Lmin)) was used to calculate the contrast values. In ring adjustment mode, the program calculated the mean logarithmic spatial frequency and the standard deviation of the 10 presentations.
To find out whether fewer presentations also produce a reliable result, the means and standard deviations were also calculated for just the first 5 presentations, disregarding presentations 6 to 10.
Contrast sensitivity values and spatial frequency values of the 10 subjects were used to calculate the geometric mean for stationary and rotating Siemens stars.
The second experiment focused on validity and repeatability of the Star-Ring Test. Ten subjects between 20 and 44 years of age participated in this trial. The average age was 29.4 ± 7.1 years. The subjects were naïve with no experiences in psychophysical testing. Five of the subjects were female and five were male. Inclusion criterion was a well-corrected visual acuity (≥0.8). As in the first experiment, the right eye was tested (left eye occluded). We compared contrast sensitivity results of the Star-Ring Test with those of the Vistech VCTS 6500 chart. The Vistech chart is one of the most common grating contrast sensitivity tests1 and widely used for clinical investigations and research.24–28 The chart contains sine wave gratings arranged in five rows (spatial frequencies 1.5, 3, 6, 12, and 18 cpd) and nine columns (contrast levels). The step sizes of contrast are irregular and vary from 0.11 to 0.37 log units. Contrasts were checked by the videophotometer LMK 98 (TechnoTeam, Germany). Mean luminance of the Vistech chart was 182 cd/m2, and the test distance was 3 m.
Adjustments of the Star-Ring Test were matched to the Vistech chart. That is, the same five spatial frequencies were used, and the test distance was again 3 m. Hence, the total extension of the Siemens star in the visual field was 4.2°. The subjects had to adjust the contrast with the five given spatial frequencies (contrast adjustment mode), whereas—according to the results of experiment 1—only rotating test patterns with a rotational speed of 20° per second were presented. In the ring adjustment mode, test patterns rotated with 10° per second and a 10% contrast level was defined. In this experiment, the number of presentations per trial was reduced to 5, according to the results of experiment 1. To examine repeatability, all 10 subjects were tested twice on two different days. The repeatability coefficient (RC) was calculated as follows according to the method described by Bland and Altman29:
where s is the standard deviation of the differences between the 2 measured values of each subject. It is defined as square root of the summed differences (d) divided by the number of subjects (n).29
Equation (Uncited)Image Tools
In this experiment, practicability of the Star-Ring Test for subjects with abnormal contrast sensitivity was investigated. Three subjects were included; their characteristics are listed in Table 1. Letter visual acuity was determined in the preceding ophthalmological examination with best spectacle correction. Contrast sensitivity of the subjects was determined by contrast adjustment mode of the Star-Ring Test and by the Vistech chart. Both test methods were used under the same conditions as in experiment 2, but the subjects were tested only once. Only one eye was tested, the other eye was occluded.
The investigations with contrast adjustment mode showed no significant difference in contrast sensitivities between the stationary and the rotating Siemens star. Table 2 shows the results of the 10 subjects and the four different spatial frequencies. All p values were calculated using the Wilcoxon signed-ranks test. Fig. 6 shows that contrast sensitivity values peak at 6 cpd and decrease with higher spatial frequencies.
Standard deviations for sets of 10 repeats were pooled and are plotted in Fig. 7. All standard deviations were calculated from the logarithmic contrast sensitivity (logCS) values and are reported in units of logCS. With the exception of 24 cpd, the standard deviations for rotating Siemens stars are lower as compared with stationary Siemens stars in the contrast adjustment mode.
Moreover, the means of the first 5 presentations of each trial were compared with the means of all 10 presentations. In 93% of all trials, the means of 5 presentations differed from the means of 10 presentations by <0.10 log units; in 57% of all trials the difference was <0.05 log units. No clear trend was noted in either direction.
The average duration of the measurement for each spatial frequency in the contrast adjustment mode was 94 s for stationary patterns and 85 s for rotating patterns (each with 10 presentations).
Fig. 8 shows the averages of adjusted spatial frequencies for five contrasts in the ring adjustment mode. Results for stationary and rotating stars are also very similar, and no significant differences were found (Table 3).
Standard deviations for sets of 10 repeats were pooled and are plotted in Fig. 9. They are calculated from the logarithmic spatial frequencies and are given in log units. Standard deviations decrease with increasing contrast and average between 0.11 and 0.05 log units. Moreover, except for a 10% contrast, the standard deviations for rotating stars were lower than the values for stationary stars in the ring adjustment mode.
The means of the first 5 presentations of each trial were compared with the means of all 10 presentations for this mode as well. In 87% of all trials, the mean of 5 presentations deviated by <0.05 log units from the mean of 10 presentations. As before, no clear trend in either direction was noted.
The average duration of the trial for 1 contrast in the ring adjustment mode was 46.5 s for both stationary and rotating patterns (10 presentations).
In this experiment, the validity of the Star-Ring Test was investigated by comparison with the Vistech chart. Fig. 10 shows the averages of the logCS estimated by the Star-Ring Test (contrast adjustment mode) and by the Vistech chart. For both test methods, the peak spatial frequency of the CSF is 3 cpd. With higher spatial frequencies, the CSF shows the typical decrease. In the range between 1.5 cpd and 3 cpd, the CSF estimated by the Star-Ring Test is flatter than that estimated by the Vistech chart (Fig. 10).
Mean logarithmized contrast sensitivities of both tests and the significances (p values) calculated by the Wilcoxon signed-ranks test are listed in Table 4. For spatial frequencies of 3, 6, and 12 cpd, contrast sensitivities estimated by the Star-Ring Test are significantly lower (difference averages 0.19 to 0.40 logCS) than those estimated by the Vistech chart. For 1.5 and 18 cpd, there is no significant difference.
The RCs for the Star-Ring Test ranged from RC = 0.22 log units to RC = 0.38 log units, depending on the tested spatial frequency. For the Vistech chart it ranged from RC = 0.34 log units to RC = 0.67 log units (Table 5). The average duration of one trial in the contrast adjustment mode of the Star-Ring Test was 67 s. For the Vistech chart, the measurement (all lines) took 101 s on average, which means 20.2 s for each spatial frequency.
In the ring adjustment mode of the Star-Ring Test, the geometric mean of the adjusted spatial frequency at 10% contrast was 24.87 cpd. The RC for rotating Siemens stars was RC = 0.13 log units. The average duration of one trial in the ring adjustment mode was 81 s, whereas the fastest trial took 29 s and the slowest 172 s.
In this experiment, three subjects with abnormal contrast sensitivity were tested with the Star-Ring Test (contrast adjustment mode) and the Vistech chart. Estimated contrast sensitivities are plotted in Fig. 11. It is shown that contrast sensitivities are reduced for all spatial frequencies in both test methods, compared with averaged contrast sensitivities shown in experiment 2. The reduction ranged from 0.28 to 0.94 log units (Star-Ring Test) and from 0.48 to 1.32 log units (Vistech chart). Subject 3 could not perceive any pattern on the Vistech chart at spatial frequencies of 12 cpd and 18 cpd.
Measurements of contrast sensitivity by means of chart-based tests usually require the subject to read aloud the optotypes seen. This may be undesirable in certain circumstances, for example, when the head of the subject is fixed for experimental purposes by means of a forehead and chin rest to look at the test pattern through an optical system in an exactly centered position. No verbal response is required in the determination of contrast sensitivity using the Siemens star. The responses are made by pressing buttons, which means head motions are eliminated during the examination.
The Star-Ring Test is performed according to the method of adjustment. This makes the test rapid but also renders the measuring result susceptible to the influence of the subjective criterion. A motivated subject may tend to adjust the contrast threshold more likely too low, whereas a cautious subject may tend to set to a position where he/she is absolutely sure to recognize the pattern. Accordingly, the test is best suited for measurements of progression or side-by-side comparison of two viewing situations (e.g., contrast sensitivity in the presence/absence of contact lenses) presuming the subjective judgment to be unchanged. Results concerning the RC of the Star-Ring Test (see experiment 2) support this statement.
Defining the threshold of recognition using the Star-Ring Test is a challenging visual task. Rotating stars are experienced to be more pleasant. Subjectively, this makes the adjustment of contrast or ring size easier. Influences of motion perception due to the rotation of the test pattern vs. the stationary pattern do not appear to play a significant role (Figs. 6 and 8). No significant differences were detected in any comparison in experiment 1. The measured values show little deviation for rotating patterns (Figs. 7 and 9). Another advantage is that local adaptation is prevented and interfering after-images are reduced.
In experiment 1 in the ring adjustment mode, the standard deviations of the respective presentations were considerably larger at low contrast (contrast <5%, Fig. 9). This may be related to the smaller slope of the CSF in this range. It therefore makes sense to use the contrast adjustment mode in which the ring sizes; hence, the spatial frequencies are given, and the contrast threshold is adjusted. Conversely, it is conceivable that the deviation increases markedly at high spatial frequencies in contrast adjustment mode because the CSF steepens increasingly in this range. (This, however, will have to be addressed in a subsequent study.) Presumably, a combination of the two modes would be useful, in which contrast adjustment mode is used for low and moderate spatial frequencies while ring adjustment mode produces more accurate results for high spatial frequencies.
Experiment 2 contained a comparison of the Star-Ring Test with a common test method, the Vistech chart. With all spatial frequencies except 1.5 cpd, the Vistech chart showed higher mean contrast sensitivities than the Star-Ring Test (Fig. 10). For 3, 6, and 12 cpd, the differences are statistically significant (Table 4). The mean CSF of the Star-Ring Test consequently is flatter. A possible reason for this is the relatively high chance level for the Vistech chart. It is a three alternative forced choice method, and so the chance level is 33% for one contrast step. This means that a test subject could give the correct answer by chance for two consecutive contrast steps with an 11% probability. The Star-Ring Test uses five decisions per contrast step and is not as susceptible to chance. Therefore, contrast sensitivity measured by the Vistech chart could be overestimated. It is also conceivable that the print quality of the Vistech chart affects the results. The contrasts predetermined by the manufacturer were approximately complied with, as detected by the LMK 98. However, the contrast across one test pattern is often not stable and can be slightly higher than intended in several areas. This principally can lead to an overestimation of contrast sensitivity in all spatial frequencies.
Another reason for differences of contrast sensitivity values could be the task of the Star-Ring Test. The contrast ought to be reduced, until there is no pattern visible inside the ring. However, subjects may tend to stop contrast reduction a bit before having reached the threshold. Thus, contrast sensitivity would be underestimated. However, at spatial frequencies 1.5 and 18 cpd, no significant differences were found comparing the Vistech chart to the Star-Ring Test. For 1.5 cpd, this is explainable with the flatter form of the CSF of the Star-Ring Test between 1.5 and 3 cpd (Fig. 10). The reason for this might again be the task. The pattern inside the ring (i.e., higher spatial frequencies) ought to be invisible. As the CSF decreases to the left, spatial frequencies lower than 1.5 cpd outside the ring are not visible either. Theoretically, a zonular part of the test pattern inside the ring comprising the maximum of the CSF ought to be visible. In practice for most subjects it is not. In doing so, the typical decrease from the maximum of the CSF to the lower spatial frequencies—called low spatial frequency rolloff—does not exist or at least is much flatter. Thus, contrast sensitivities of both tests converge.
The results for 18 cpd also show almost identical values in both tests. Using the Star-Ring Test at 3 m viewing distance for measuring 18 cpd, one sinusoidal period is displayed by 10 pixels. This means that the luminance range of one period is resolved into 10 increments, which appears to be adequate for a proper sinusoidal design. Compared with the Star-Ring Test, Vistech chart results are higher in the midrange spatial frequencies, and there is a stronger decay at the higher spatial frequency end of the CSF. This might indicate that the aforementioned typographic deficiencies influence results especially in high spatial frequencies.
The peak of the mean CSF was determined at 3 cpd (both by the Star-Ring Test and by the Vistech chart) in experiment 2. In experiment 1, it was identified at 6 cpd by the Star-Ring Test. The peak of the standard CSF is located in between these two spatial frequencies, as demonstrated by Souza et al.30
There are only marginal differences between the final results of 5 vs. 10 presentations, as shown in experiment 1. The test routine therefore was down sized to 5 presentations per spatial frequency in experiments 2 and 3. This allows for a reduction of measurement periods and subjective strain, especially if a large number of trials is needed.
In experiment 2, comparison of the Star-Ring Test and Vistech chart shows a better repeatability of the Star-Ring Test. RC of Star-Ring Test ranges from 0.22 to 0.38 log units, whereas RC of the Vistech chart ranges from 0.34 to 0.67 log units (Table 5). These values are consistent with previous studies. Pesudovs et al.,31 for instance, determined RC values from 0.30 to 0.85 log units for the Vistech chart. A possible reason for the poor reliability of the Vistech chart is the use of large contrast increments (averaging ∼0.25 log units), single trials at each contrast level,32 and a high level of chance, as discussed above. For the FACT, RC values from 0.30 to 0.75 log units have been reported.31 Letter tests such as the Pelli-Robson chart (RC value from 0.18 to 0.20 log units)6,33,34 and the Mars Letter Contrast Sensitivity Test (RC value from 0.121 to 0.20 log units)6,33,34 show better repeatability.
As mentioned above, there is an influence of the subjective criterion on the results of the Star-Ring Test. Performing repeated measurements or measurements of progression, it can be assumed that subject's criterion remains unchanged. It appears to have been so in this experiment; otherwise RC values would have been negatively impacted. For the repeatability of the Star-Ring Test, it is beneficial to adjust the contrast several times, minimizing deviations from incorrect measurements. Moreover, the result is not subject to the influence of level of chance. The ring adjustment mode also was found to be highly repeatable (RC = 0.13 log units).
Comparing the repeatabilities of both modes of the Star-Ring Test, RC in the ring adjustment mode is considerably smaller (0.13 log units) than in the contrast adjustment mode (0.22 to 0.38 log units). As discussed above, it must be noted that different ranges of the CSF were examined. On the one hand, the ring adjustment mode was used to determine resolvable spatial frequency in the steep range of the CSF (for a 10% contrast). On the other hand, contrast adjustment mode was applied to examine contrast sensitivities in the middle range of the CSF (3 cpd to 24 cpd). If the ring adjustment mode is used with smaller contrasts (1.25% and 2.5% in experiment 1) larger standard deviations for 10 repeats will be detected than with higher contrasts (Fig. 9). Thus, for the subjects it appears to be more difficult to mark the limit of resolution with a low contrast of the test pattern. Accordingly, the RC values are higher in this case. Therefore, the ring adjustment mode may not always be the better choice; however, in the range of high spatial frequencies, it appears to be more advantageous compared with the contrast adjustment mode.
A comparison of measurement periods in the 2 modes in experiment 1 shows that ring adjustment requires approximately half as much time (46.5 s) as contrast adjustment (85 s for rotating and 94 s for stationary stars). Adjustment of the ring size appears to be the task more easily carried out by the subjects, each of whom had experience in psychophysical testing. As expected, the naïve subjects in experiment 2 needed more time for a trial. In the ring adjustment mode, they needed even considerably longer (81 s in average), although, in contrast to experiment 1, only five presentations per trial were presented. Some subjects performed this test with extreme care (this considered, a time limit might be useful). However, a positive effect of that is the very good repeatability.
In experiment 3, contrast vision of three elderly subjects with incipient cataract was investigated. A reduction of contrast sensitivity was detectable with both test methods, the Star-Ring Test and the Vistech chart. The reduction was caused by the opacity of the lens on one hand, but by normal age-related changes of contrast sensitivity on the other hand. With the Vistech chart, the measured loss of contrast sensitivity tends to be larger than with Star-Ring Test, compared with the CSFs in experiment 2. A possible reason might be that—although the Vistech chart requires a forced choice mode—the elderly subjects did not risk guessing and chose to abort a line when no clear pattern was visible. This may lead to an underestimation of contrast sensitivity. However, with the young healthy subjects in experiments 1 and 2, the forced-choice procedure worked well. Subject 3 did not recognize even the highest contrasts of the Vistech chart at spatial frequencies 12 and 18 cpd. With the Star-Ring Test, contrast thresholds for all five spatial frequencies were determined, as contrast can be adjusted up to ∼90%, depending on the monitor.
Comparability of the Star-Ring Test and the Vistech chart is limited to some degree because of the different test design. Vistech uses discrete steps, whereas the Star-Ring Test uses a continuously variable measure. Thus, direct comparison of contrast sensitivity and measurement error between the 2 tests must be regarded with caution. In a further study, it would be reasonable to test the performance of the Star-Ring Test against a monitor grating contrast sensitivity test without discrete steps, which is commonly used in vision-research laboratories.
It is a general advantage of the Star-Ring Test that contrast is variable in a wide range, unlike Vistech or FACT, which have a limited contrast spread. Another advantage is the variability of the test distance. It is customizable in the settings of the software and thus feasible for different test setups. Printed charts are subject to wear and tear, which might lead to a change of contrast. It is not so with a computer monitor. Initial contrast calibration and occasional recalibration maintains constant quality.
Letter-based contrast tests such as Pelli-Robson-Chart or MARS-Letters are very easy for subjects to understand because reading letters is a very common task. The results provide a notion about contrast sensitivity in everyday situations. Nevertheless, letters do not have a defined spatial frequency. Those tests are not feasible for measuring the entire CSF. The Vistech chart evaluates contrast vision with five defined spatial frequencies. A best fit CSF can be derived. On the other hand, repeatability is limited and the level of chance is high. The Star-Ring Test is appropriate for measuring different spatial frequencies with a very low level of chance and significantly higher repeatability.
In the Star-Ring Test, a black, sharply delimited ring is used to mark the resolution threshold. However, the high contrast of the ring might lead to some interference with the underlying spokes of the star and thus have an impact on the measuring result. It is conceivable that this is partly responsible for the smaller contrast sensitivity values in middle spatial frequency range compared with the Vistech chart. In the contrast adjustment mode, ring diameter in the visual field varies between 0.52° and 1.05° at a test distance of 6 m and between 1.05° and 2.1° at 3 m, respectively. Although the position on the monitor changes, subjects use about the same area on the retina to perceive it. Thus, an effect of ring size on the adjusted contrast threshold is improbable.
An interesting phenomenon called spurious resolution occurs when a subject with incorrect correction views the Siemens star. It causes light stripes to be perceived as dark stripes and vice versa in certain ranges of spatial frequencies. The pattern appears phase-inverted (Fig. 12). This is caused by defocusing which refracts the light to the extent that neighboring brightness peaks, superimposing on an originally dark area, and vice versa.35 This phenomenon occurs with all periodical patterns. Therefore, the test is suitable for contrast testing with good correction, because contrast sensitivity may otherwise be over-assessed.
Because of the given gray levels of the monitor (256 gray levels), the display of contrast is limited. Michelson contrasts of <0.4% cannot be displayed (depending on the monitor). In some cases, contrast threshold may be lower and detract from exact determination. The Star-Ring Test does not use discrete contrast value graduations, and the repeatability was demonstrated to be high. As a result, the test is capable of detecting even subtle changes of contrast sensitivity.
The Star-Ring Test is a sensitive contrast vision test producing reliable results in little time, which renders the test convenient for laboratory investigations. It is easy to understand, and the interactive mode ensures that attention level of the subjects is high. This makes it feasible even for eye care practitioners. The CSF derived from a Star-Ring Test procedure is very similar to that of the Vistech chart. The tests allow for a non-verbal examination. Initial results and experiences obtained with the Star-Ring Test are promising. The new test method appears to be also useful for elderly subjects with eye diseases.
We thank Halle Eye Laser Centre for support in recruitment of subjects with eye diseases. This study was supported by German Federal Ministry of Education and Research (BMBF), grant 01EZ0608.
Course of Optometry
University of Applied Sciences Luebeck
Moenkhofer Weg 239, 23562 Luebeck
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