A simple light meter, measuring in foot-candles or lux. Records the reading along with an image from the front-facing camera.
The prerequisite for plant health is light but when growing them indoors, light levels vary immensely. When the sun is shining directly on a plant, that's considered "direct sun", but the majority of your indoor plants prefer "bright indirect light". Without a light meter, it's difficult to judge what's bright and what's low in terms of indirect light. A light meter will help you put a quantity to an otherwise, subjective understanding of brightness.
Having the camera output shown at the same time will help you link what is in the physical environment to the brightness level.