Find sensor used for Grayscale

I was instructed to do a grayscale detector.......What sensor should I use???

I would consider an optical based sensor would be appropriate.

Lefty

use a colour sensor and use only the green (part of the) signal

Arguably the finest spot light meter ever made is the Asahi Pentax 1 Degree Analog Spotmeter V. Ansel Adams used one. It uses cadmium sulfide sensors.