Devices that are instruments have a definite relationship between output and detection.
Devices such as temperature sensors, voltage detectors, etc can be relied upon to produce a known result.
Most of the devices you are talking about have a wide variation between one batch and the next. They are not even sorted into groups – like transistors or LEDs.
You may be able to find a graph of the relationships you are talking about but they are generally not used as an “instrument.”
Most of the time they are used to detect either brightness or lack of illumination and the circuitry is adjusted to operate at a particular level of brightness, for example.
You just need a single transistor to amplify the output of the photocell. I simply connect a solar cell to a digital multimeter and it produces a perfect result.
The result is always the same, with the same LEDs, same current and same distance.
When the solar cell is moved closer, the reading is higher, but in my case I require the distance to be same in all cases, as I am altering the illuminating medium.
Light meters are fairly expensive, but some digital cameras have a built-in lux meter with a readout on the screen.
This is the best approach.