This is a set of black and white bars gradually varying in contrast in 10% steps.
One set has grey bars on a white background; the other has grey bars on a black background.
Print out some sheets and give them to the patient
The patient places a sheet under the lamp during the remote session, providing a reference of known contrast
Each sheet has a number indicating the approximate contrast. Note that white paper is not perfectly white, so these numbers are approximations.
A note of caution. Printers create shades of grey using black dots of varying spacing. With ink-jet printers, these dots blend together, but it is possible to see individual dots with some laser printers. A good camera held closer to the sheet may see the individual dots, in which case the grey bars may be seen as pure black and white. This issue can be avoided by increasing the distance between the sheet and the camera, or using an ink-jet printer.