You check letter size by measuring it with a ruler. Nothing else proves the size is right. A digital acuity screen shows a calibration square of a set size. You hold a ruler against the screen and measure that square. If it is off, you use the screen's own size adjustment to correct it. Only then do the letters match the acuity they claim. An image can look crisp and still be the wrong size, which is why looking at the screen is not enough.
Two settings work together. The first is the chart-to-chair distance, which you measure and enter into the unit. The second is the letter size, which the ruler confirms. Older bulb projectors used the same idea. You measured the distance from the chair to the screen, set the chart to 20/200, and measured the height of the big E with a ruler against a chart supplied by the maker. If the height was wrong, you moved the projector tube to change the size. Get size wrong and every acuity you record is wrong too. For the projector itself, the textbook adds simple care rules: never open the lamp compartment while it runs, do not remove lenses from the barrel, and replace a dimming bulb so acuity testing stays accurate. A projected image that fades on one side of the screen is a warning that the bulb is about to fail.
Trust a measurement, not your eye. Bring a ruler to any chart calibration, and write down what you found. Recorded acuity is only as good as the chart that produced it. Keeping acuity equipment accurate is a core part of the COA instrument maintenance content area.
Incorrect Answers
- A. Brightness changes contrast, not size. A sharp, bright letter can still be too large or too small.
- C. A digital screen chart has no separate projector and screen, so this distance does not exist to check.
- D. The screen size on the box is the size of the whole panel, not the height of a letter shown on it.
Testing Pearls
- Confirm letter size by measuring with a ruler, not by eye.
- Also measure and enter the chart-to-chair distance.
- On a bulb projector, set 20/200 and measure the big E against the maker's chart.
- A dimming or half-dark image means the bulb is near failure; replace it.
- Never run a projector with the lamp compartment open.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 22, Care of Ophthalmic Lenses and Instruments.
