Intensity

Light intensity is a description of the level of a light’s brightness. The intensity of light falling on a subject can be measured using a light meter. This is called an ‘incident reading’. A light meter built into a camera does not directly measure the intensity of light falling on the subject but the level of light reflected from it. This is called a ‘reflected reading’.

Reflectance

Mark Galer

Regardless of the intensity of the light falling on the subject different levels of light will be reflected from the subject. The amount of light reflecting from a surface is called ‘subject reflectance’. The levels of reflectance vary according to the color, texture and angle of the light to the subject. A white shirt will reflect more light than a black dress. A sheet of rusty metal will reflect less light than a mirror. In all cases the level of reflectance is directly proportional to the viewpoint of the camera. If the viewpoint of the camera is equal to the angle of the light to the subject the reflectance level will be greater. The level of reflected light is therefore determined by: ~ Reflectance of the subject. ~ Intensity of the light source. ~ Angle of viewpoint and light to subject. ~ Distance of the light source from the subject. Although the intensity of the light source may remain constant (such as on a sunny day) the level of reflected light may vary.

As the distance between the subject and the light source increases the level of light illuminating the subject decreases. The amount of light falling on a subject decreases to 25% of its original intensity when the light to subject distance is doubled. This change in the level of illumination is called fall-off and is quantified by the ‘Inverse square law’ (observe the falloff in the image below). For example, if a reading of f16 is obtained when the light to subject distance is one metre, at two metres the reading would be f8, at four metres f4. These rules do not change regardless of the light source. Although fall-off does not present a problem when working with direct sunlight (all subjects being the same distance from the sun), it does need to be considered with reflected light, window light and artificial light sources. The visual effect of subjects at differing distances to the light source is uneven illumination.

Inverse square law

When a surface is illuminated by a point source of light the intensity of the light at the surface is inversely proportional to the square of its distance from the light source.

Daniel Tückmantel

Activity 3

Using a floodlight or window light, illuminate two people standing at one and two metres from the light source. Position the people two metres further away from the light source so that the people are now three and four metres from the light source. Does the difference in brightness between the two people illuminated by the light source increase or diminish as the light source moves further away from the subjects?