what is a grey card?

you might want to read the previous article: what's special about grey?

you might also be interested in these articles: where is middle grey? | exposure value

Before reading this article you should be familiar with the article entitled "what's special about grey?". That article lays the foundations for two unique properties of grey:

1. it reflects back a fixed proportion of the incident light
2. it does so without changing the colour of the incident light

Read more: what is a grey card?