Even with the definitions above, the appropriate application of irradiance or radiance measures is not always clear. The rule is that if the direction or position of the light source is important, then radiance measures are used. If the position of the light source is not important, then irradiance measures should be collected. A few examples below may help clarify this issue further.

Table 1

Measurement of electromagnetic energy within the optical spectrum, which includes ultraviolet radiation, visible light, and infrared radiation. An ideal radiometric detector has a "flat" spectral response.

 Irradiance Radiance • Measure of radiant energy from all • Measure of radiant energy viewed directions over a 180° field of view. from a specific direction or region in space. • Common units: • Common units: - erg/s/cm2 - erg/s/cm2/sr - ^W/cm2 - ^W/cm2/sr - photons/cm2/s - photons/cm2/s/sr

Photometric measures

### Photometric measures

Measuring human visual responses to radiant energy. A measurement of visible light that falls between the wavelengths of 400-700 nm. The spectral response of a photometric detector is not flat but attempts to reproduce that of the average human eye. Two average human-eye responses are used: a photopic response (maximum sensitivity at a wavelength of 555 nm), and a scotopic response (maximum sensitivity at a wavelength of 507 nm). By convention, photometric measurements are considered photopic unless otherwise stated.

Illuminance (illumination)

Luminance

Measure of light from all directions over a 180° field of view.

Common units:

Measure of light viewed from a specific direction or region in space.

Common units:

A radiance measure would be appropriate to determine the amount of light passing through a hole in the forest canopy and falling onto a basking lizard. However, having said that radiance is good for measuring the amount of light in a particular direction of space, this measure is not an appropriate unit for characterizing point sources of light (e.g., light emitted from a laser) because the field of collection of the detector usually extends outside the area of the

Fig. 1. The "ideal" spectral responses of (A) a radiometric detector that has a relatively flat spectral response (sensitive to all wavelengths equally) and (B) the sensitivity of a photometric detector (e.g., luxmeter) that has a spectral response approximating the spectral range of human photopic color vision. Human scotopic sensitivity is also shown for comparison (see Subheading 3.4. and Table 1 for details).

Fig. 1. The "ideal" spectral responses of (A) a radiometric detector that has a relatively flat spectral response (sensitive to all wavelengths equally) and (B) the sensitivity of a photometric detector (e.g., luxmeter) that has a spectral response approximating the spectral range of human photopic color vision. Human scotopic sensitivity is also shown for comparison (see Subheading 3.4. and Table 1 for details).

point source. For this reason it is better to use an irradiance detector and direct the point source 90° to the collection surface.

A common problem in circadian rhythms research is the characterization of a phase-shifting light stimulus. For example, when a light pulse is delivered to a rodent in circadian experiments, it is often placed into a chamber designed to provide near-uniform lighting (such a device is shown in ref. 6). Light usually originates above the animal, and is scattered using a frosted glass screen (an "opal"screen). As a result, the light falling on the animal is uniform and occupies an extended area. In this example the light should be characterized using a

Fig. 2. Representation of the arrangement of the photocell used for the detection of (A) irradiance/illuminance and (B) radiance/luminance. Irradiance/illuminance measures collect radiant energy from all directions over a 180° field of view, whereas radiance/luminance measures collect radiant energy viewed from a specific direction or region in space (see Table 1). In most cases a lens is used to determine the collection angle for a radiance detector; however, a narrow aperture tube can be substituted (as shown in B). Figure based on that shown in ref. 21.

Fig. 2. Representation of the arrangement of the photocell used for the detection of (A) irradiance/illuminance and (B) radiance/luminance. Irradiance/illuminance measures collect radiant energy from all directions over a 180° field of view, whereas radiance/luminance measures collect radiant energy viewed from a specific direction or region in space (see Table 1). In most cases a lens is used to determine the collection angle for a radiance detector; however, a narrow aperture tube can be substituted (as shown in B). Figure based on that shown in ref. 21.

measure of irradiance with the detector head placed either on the opal screen or at the level of the animal. A final example would be the measurements of sunlight. Sunlight within the environment varies in its direction, spectral quality, and flux (21). For example, if a measure of all the available light arriving on the ground is required, then irradiance measures are appropriate. By contrast, if measures of sunlight at the horizon are required (the viewing direction is clearly important), then radiance measures are appropriate. In this case the radiance measure would ignore much of the scattered light above the horizon.