How do we measure darkness?

Related questions

1. What is CIE A level?
2. Is CMYK A color space?
3. What color space is best for printing?
4. What is CIE RGB?
5. What is the unit of color?
6. Is light stronger than darkness?
7. What are A levels equivalent to?
8. Is Red heavier than yellow?
9. What is TCU in water?
10. What is full meaning of etc?

Tags

1. #optics
2. #coefficient
3. #units
4. #inverse
5. #number
6. #light
7. #standard
8. #matter
9. #determine
10. #length
11. #measured
12. #absorption
13. #absorbs
14. #which
15. #darkness