The book has a high graphics content, usually with a text on one page and explanatory diagrams on the opposite page. Below are two samples from Chapter 3, "Luminance".

How much light is reflected from a surface depends on the color of the surface. A dark grey wall may receive the same flux as a white wall but it will reflect less light. The reflectance of a Lambertian surface is a number between 0.0 and 1.0.

Reflectance is worked out using the flux arriving at a surface compared with the flux leaving a surface. See the diagram opposite. Note how not all the light which issues from the luminaire arrives a the surface. We consider only the lumens in that distorted pyramid. (This is a good example of when we are interested in only a part of the flux from a luminaire, i.e. the solid angle of part of a sphere, not the whole sphere.)

So it is lumens out divided by lumens in. Since all normal surfaces reflect some light reflectance is never quite zero. And no surfaces reflect all of the incident light the reflectance can never quite be unity.

It is important to remember that we are talking about the quantity of light, i.e. the flux measured in lumens.

Samples from chapter 2. Samples from chapter 4.