The measurement of quantities associated with @[email protected] The quantities may also describe the variation of the energy with respect to other variables such as @[email protected], time, position, direction (solid @[email protected]), area normal to the light or projected area of emitting or receiving surfaces. If the light is monochromatic, it is sometimes convenient to replace the @[email protected] by the corresponding number of photons (or quanta) which is obtained by dividing the energy by $$\frac{h\ c}{\lambda }$$ where $$h$$ is the @[email protected], $$c$$ the velocity of light and $$\lambda$$ the @[email protected] of the light.