4 Introduction to Lighting Intensity
Light intensity (also referred to as brightness, or luminance) is measured in Lumens. As light travels from its source, each doubling of distance results in a fourfold reduction of light intensity. This is known as the Inverse Square Law. This is an especially important consideration when choosing the power (Wattage) of lamps when lighting large areas.
The nature of the inverse square law of light can be exploited in two ways. Soft, yet dramatic modelling light with lots of highlights and shadows can be achieved by bringing a soft light source very close to the subject, so that light on one side of the subject is four times stronger than that falling on the opposite side. Conversely, if a moving subject needs to be lit evenly, by using a distant light with a focussed beam the distance travelled by the subject is a fraction of the distance between the subject and the light source. Therefore the light is relatively even in intensity, even though the subject moves towards the light source.
4.1 Shaping Light
The properties of light can be altered in a number of ways. Firstly, many lamps can be mechanically adjusted from ‘spot’ to ‘flood’ (narrow beam to wide beam, respectively) by repositioning the lamp within its reflective bowl. These are normally incremental adjustments, with the spot position providing the more concentrated light effect. In the flood position, the light will be spread to a wider area.
Further adjustment to the direction of the light emitting from the lamp can be achieved with ‘Barn Doors’ (large metal flaps) which attach to the lamp head, or by adding other devices to the lamp such as ‘Snoots’ (a cylindrical attachment for preventing light spilling sideways). In addition, a mouldable black-coated aluminium sheet called ‘Black Wrap’ can be attached to barn doors to stop light spilling on to specific parts of the subject that you want to remain in shadow.
4.2 Soft Light & Hard Light
Soft light, so-called because ideally it will cause no shadows, is created by the use of ‘Diffusers’. The most common diffusers are either thin opaque plastic sheet diffusers, or non-woven polyester ‘spun’ diffusers. Both are made from heat resistant materials and are temporarily attached to barn doors with clips. Not only do these diffuse the light and give softer edges to shadows, but they will also reduce the light’s intensity.
Hard Light is the exact opposite to soft light. It is the light from the lamp free from all diffusion (un-diffused light) and, similarly, is so called due to the hard edge of the shadow created by the light.
4.3 Filters
Filters such as coloured glass filters or ‘Gels’ can be attached to lamps to change the quality of the light and/or colour temperature. A gel is a thin sheet of transparent heat-resistant material, usually made of polyester or polycarbonate, which is attached to the front of the lamp. Gels have a limited life, so glass filters are sometimes preferable.
Tungsten conversion filters (such as ‘Dichroic’ filters) can effectively raise the colour temperature to match daylight. A more common method is to use blue gels, known as ‘CTB’ (Colour Temperature Blue). These gels are cheaper to buy than dichroic filters and come in various strengths, thus making it easier to accurately match the daylight colour temperature. You can also obtain daylight conversion gels, known as ‘CTO’ (Colour Temperature Orange), to attach to external windows to reduce the daylight temperature to that of tungsten lamps. Various levels of ‘Neutral Density’ (ND) filter gels can also be added to lamps and windows. An ND filter has the effect of reducing the intensity (brightness) of a light source – without altering its colour temperature.
Coloured gels simply add a particular hue (colour) to the light, and are usually employed to achieve a particular artistic colour wash to a scene. Again, these are heat resistant and attach to the lamp’s barn doors with clips. To get the full effect of these colour gels, the camera’s colour temperature should be set manually before the gel is attached to the lamp. Gels must be used with caution as once the shot has been recorded it cannot easily be rectified in post-production.
4.4 Reflected Light
Reflected Light can also be used to great effect and can be utilised by simply bouncing lights off available surfaces such as a ceiling or wall – though care should be taken as the reflected light will take on the colour of the surface.
Another method is to use specially made light reflecting panels. These reflecting panels will normally have one side with a matt white finish and another with a highly reflective silver finish. In some instances, using reflected light can be a convenient way to light a subject from another angle without introducing an additional light source. Any reflected or or ‘bounced’ light is, by its very nature, soft light.