In Hess-Lancaster testing, what color light does the examiner project onto the screen to be matched?

• A. The green light
• B. The red light
• C. No light
• D. A blue light

Answer: (B)

In Hess-Lancaster testing, a red light is projected onto the screen to be matched. This setup uses red-green glasses so that each eye can be tested separately: the red-filtered eye sees the red projection, while the green-filtered eye sees little or nothing from that same red light. By having the patient match the position of the red light, the examiner can map how each eye moves and where misalignment occurs. The red projection is chosen specifically because red wavelengths pass through the red filter but are blocked by the green filter, allowing clear separation of the images seen by the two eyes and enabling accurate motility mapping.