The light meter and spectrometer are two fundamental tools for assessing lighting conditions on both photography and film sets. Though used in both situations, a distinction lies in the importance of their use in each. Indeed, however useful they may be to a photographer, he or she is not obliged to use them, while their use is pretty much mandatory for a cinematographer.
1. Light meter
A light meter is an instrument used to measure the amount of light, giving data that, in the case of photography, can be used to set camera exposure, sensitivity, shutter speed and aperture. A light meter may be incorporated in your digital camera, or may be a stand-alone, hand-held device. Internal and external devices have different features, but, before we talk about this, let’s first understand the general principle behind the instrument.
When lighting a studio or set, the first step is to use the light meter to measure the luminous flux in order to set the right exposure for a better shot. The main, or key, light can be measured in two ways, one considering incident light and the other reflected light.
Incident light is that which directly illuminates the subject. To measure its intensity, bring the light meter close to what you want to photograph and point its photocell sensor in the direction of the light source, so the measurement is not influenced by the light reflected from the subject itself. Then, just read off the measurement on the device.
1. Light meter positioned between the light and subject, with the photocell facing the source
2. Light beam illuminating the subject
With exactly the same lighting conditions as the first measurement, reflected light can then be measured at the camera position, with the light meter facing the illuminated subject. The device will almost always give a different result from the first measurement. Indeed, the incident light reading corresponds to the amount of light projected by the light source onto the subject, regardless of what it is, while the reflected light value will vary with different subjects (e.g. a dog, landscape or person), as it is reflected in different ways.
For example, the darker a person’s skin is, the less light is reflected, as more light is absorbed. The light meter thus indicates less light, suggesting a greater aperture or slower shutter speed, in order to let in more light. The opposite happens with whiter skinned models. This is why reflected and incident light give two completely different exposure readings. In general, light meters are calibrated to light reflected from an 18% gray piece of cardboard. Values varying from 18% are then used to suggest the best aperture and exposure settings.
1. Light beam hitting the subject on the right side
2. Reflection of the light beam from the subject towards the light meter
Integrated and external light meters
Many cameras also measure reflected light, using an imperfect algorithm to suggest an exposure setting. The latest digital cameras also use so-called ‘zebra patterning’ to indicate areas of the shot where light is over or underexposed. While this works to a certain extent, the ratio between key light and fill light, indicating the contrast between light and shadow, cannot be determined precisely in this way. That’s something only an external light meter can do. Therefore, it is simply essential for a cinematographer to use an external light meter.
2. A spectrometer can detect various light beam characteristics
Despite using a light meter to set the right exposure, you can get back to the studio and find you have distorted video colors, even if you checked the colors directly on the screen of your digital camera. Why does this happen? Well, light seemingly white to the human eye may not necessarily be so, and may acquire, for example, a greenish hue in video footage. This is because our eyes and brains adapt to various conditions in order to approximate light as white. Therefore , another essential instrument for a full lighting assessment is a spectrometer.
This serves to measure the light’s spectrum and identify its color temperature. Among the data it can provide is the Color Rendering Index (CRI), indicating the balance of colors making up white light. Suffice it to say that the higher the CRI, the more natural the result will appear. Indeed, sunlight has a uniform spectrum and a CRI close to 100. Another fundamental indication a spectrometer can give is color dominance, which can be corrected with the right filter. A neon that appears white to us, for example, often tends to green from the point of view of a camera. It goes without saying that lighting a subject with greenish neon light is not ideal, unless you have that precise effect in mind.
The spectrometer uses an optoelectronic sensor to perform the measurement, and the most useful values indicated are Correlated Color Temperature (CCT), Light Intensity (lux), Color Rending Average (Ra, analogous to CRI), and Chromaticity (CIE, color coordinates in a chromaticity diagram).