Most poultry managers use the conventional light meter (photometer) when attempting to ascertain light intensity (footcandles or lux) in a house or when trying to equalize intensity among several houses. This is acceptable when only one type of lamp is used to provide supplemental light. However, more than one type of artificial light source may be used in current light management practices. The photometer has limitations in these situations. Light intensity measurement by the light meter is not accurate when comparing different light sources.

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The photometer, which displays intensity as footcandles, has a light receptor cell that perceives light with the same spectral intensity as the human eye. Maximum reception of light energy occurs at a wavelength of 555 nanometers (green light) and decreases to a minimum at the two extremes of the visible light spectrum (blue and red) as shown in Figure 1. This results in an uneven detection of the light energy measurement across the visible light spectrum.

The visible light spectra of supplemental light sources vary significantly from one another and thus the sensitivity of a photometer to these light sources is not the same. Figures 2, 3, 4, and 5 show the light spectra of incandescent, high pressure sodium, cool white fluorescent and deluxe warm white fluorescent.¹

Absorption of photons is required for any light induced effect to occur and should be considered as a standard unit of measure for light intensity. This is especially true where light sources other than the incandescent lamp is used. 2,3,4 and intensity comparisons between houses is desired. The instrument required is more expensive than the photometer and will be impractical for most poultry operations. The measured intensity relationship within a light source between footcandles and photons is proportional and linear. Thus, a relationship between light intensity photons (M/sec/m²) and footcandles (fc) can be determined. If the poultryman is to achieve equalized intensity levels with different light sources, the most sensible unit to standardize light intensity measurements is the photon (M/sec/m²).

A correction factor to equate light intensities as M/sec/m² per footcandle for each light source was arrived at by:

Correction Factor = (M/sec/m² for source) / (fc for source)

The correction factor calculated for each light source is shown in Table 1.

Table 1: Correction Factors for Various Light Sources to Equalize the Number of Photons from Footcandle Data.
Light Source	Photons per Footcandle
120 Volt Incandescent	.215
130 Volt Incandescent	.222
High Pressure Sodium	.142
Cool White Fluorescent	.154
Cool White Deluxe Fluorescent	.202
Warm White Fluorescent	.152
Warm White Deluxe Fluorescent	.190
Day Light Fluorescent	.167
Vita Light Fluorescent	.196
Biaxial Compact Fluorescent (2700K)	.144

It becomes apparent that in a practical measurement situation where light intensity is equalized in footcandles for incandescent and any other alternative light source, a higher light intensity in photons (M/sec/m2) will be observed in the incandescent than any of the other light sources. If intensities are equalized in terms of footcandles, these results demonstrate that some of the energy efficient light sources can reduce actual light intensity in photon energy by approximately 50% if incandescent light sources are replaced.

To replace one light source with another and still maintain the same light intensity as photons, a correction factor for a specific light source (Table 1) can be utilized in an algebraic expression to determine the needed light intensity.5 In determining the desired light intensity, us the following equation with the information in Table 1:

fc needed for NLS = (fc of CLS) (CF of CLS) / (CF of NLS)

NLS = New Light Source CLS = Current Light Source
CF = Correction Factor fc = Footcandle

When controlled environmental housing is used in conjunction with low intensity lighting, it is critical that at least the minimal threshold light intensity be attained. There is no conclusive evidence for an absolute threshold intensity. It is suggested that one maintain an intensity of two to five footcandles. Using different light sources will require attention to the measurement of the light intensity in the poultry house when making comparisons between houses. When different light sources are used. This method to standardize light intensity values will be valuable to breeder managers to accommodate wavelength bias of the light meter associated with different light sources.

Source: North Carolina State University Cooperative Extension Service - October 2003