Contrast Sensitivity
Letter contrast chart with triplet scoring, built on Pelli–Robson principles.
For educational purposes only. A teaching and screening aid, not a medical device; it does not replace a standard clinical examination.
How to use
- Set the screen to full brightness, and turn off auto-brightness, night shift and any blue-light filter. Keep room lighting even, with no glare on the screen.
- Seat the patient at the chosen distance (1 m is the standard; 50 cm keeps the same letter size in degrees for a small screen), with their distance correction, testing one eye at a time.
- Press Test patient: it shows one triplet at a time, at true size, starting where the patient has got to. Use ◀ ▶ to move between triplets. Ask the patient to name each letter, and encourage guessing when the letters become faint. Accept an O for a C. Stop when two or three letters of a triplet are missed.
- Score on the score sheet, which is laid out like the chart: tap the LAST letter the patient read correctly and every letter before it is marked read, then tap any earlier letter they missed. In Test patient, the three numbered buttons toggle the letters of the triplet on screen; hold “Letters” to see which letters they are.
How it is scored
- Letter by letter: each letter read correctly is worth 0.05 log units, with the first triplet as the zero point, so log contrast sensitivity = 0.05 × letters correct − 0.15. Triplets above the first one scored, and any triplet skipped above one that was passed, are credited as read.
- The traditional triplet score — the log contrast sensitivity of the last triplet in which at least two of three letters were read — is shown alongside.
- Letters are drawn with ordered spatial dithering between two adjacent grey levels so that every triplet can be rendered on an 8-bit display. The contrast actually produced is checked for each triplet; any triplet the display cannot render within 10 % is withheld and caps the score.
Limitations
- This is an original OcuLearning chart built on the published Pelli–Robson design principles, not the Pelli–Robson chart itself. Published Pelli–Robson norms do not transfer to it, so no normal range is applied.
- Contrast on a screen assumes the display follows the sRGB standard, which uncalibrated displays only approximate. Without a photometer, the low-contrast end of the chart is the least certain.
- Use the same screen, brightness and lighting to compare results over time.
References
- Pelli DG, Robson JG, Wilkins AJ. The design of a new letter chart for measuring contrast sensitivity. Clin Vision Sci 1988;2:187-199.
- Elliott DB, Bullimore MA, Bailey IL. Improving the reliability of the Pelli-Robson contrast sensitivity test. Clin Vision Sci 1991;6:471-475.
- Mäntyjärvi M, Laitinen T. Normal values for the Pelli-Robson contrast sensitivity test. J Cataract Refract Surg 2001;27:261-266.
- Pelli DG, Bex P. Measuring contrast sensitivity. Vision Res 2013;90:10-14.
- IEC 61966-2-1:1999. Default RGB colour space (sRGB) — the transfer function used to compute grey levels.
For educational purposes only. The Clinical Exam tools are part of an educational platform. They are screening and teaching aids, not certified medical devices: a result depends on the screen (letter sizes are estimated for the type of device), the lighting and the testing distance. They are not a diagnosis, and any abnormal or doubtful result should be confirmed with a standard clinical test.