As a part of volunteering with the GMB in Honduras, I was asked to assist in the optometry station by operating a Bark Auto Refkemerator. I had to line up the “camera” of the device with the eye until a white halo appeared and I could hit the button in the middle of the joystick.

After I clicked that button 3 values appeared, an S, C and A value. I didn’t need to know what those values meant, just had to write them down for the doctor but now I’m curious.

The A value was normally a large number, while the S and C value bounced around between small single digit numbers (both positive and negative). One patient who had glaucoma in her left eye had an S value of -8.

My question being, what do these values correlate too? How do optometrists use these values to determine a prescription or diagnose them with a certain condition?