"Seeability" is a non-technical term, which describes how well the eye sees. It includes the ability to define form, which is a primary function of the eye, but it also includes color discrimination and color rendering. The functioning and reaction of the rest of the body is greatly influenced by these various factors which make up seeability. Footcandle levels, therefore, while a measurement of light quantity, are not the sole indicator of seeability. There are also measuring methods for determining light quality, such as Color Rendering Index (CRI).

 

About the Human Eye: The human eye evolved over miIlions of years in natural light of up to 10,000 footcandles and with the best color rendering of objects (CRI = 100). During this evolutionary period, the eye was generally used for focusing on large objects at fairly long range. Slight visual deficiencies did not matter as much then, so there was little or no eye strain and fatigue.

 

In today's urban society, the human being has moved indoors where light levels are usually below 100 footcandles, and coloring rendering is much poorer. (Cool White CRI 68). The seeing task has changed from focusing on large objects at long range to focusing on small symbols and objects at close range.

 

The eye suddenly has had to compensate for all these changes. Slight visual deficiencies can no longer be tolerated because of eye strain and fatigue. Optical correction with glasses has become necessary.

 

The human eye has been forced into a condition of constant compensation for the varying focuses and light levels of modern life. This constant compensation often results in eye strain: a message is sent to the brain, producing headaches, body stress, fatigue, and possibly other symptoms. It is important, therefore, that good interior lighting practice include the right quantity of light (footcandles) as well as the right quality of light (CRI).

 

The use of Vita-Lite fluorescents, which simulate the ultraviolet and visible spectrum of natural light, eliminates the eye's need to adapt to a new color of light as one goes from outdoors to indoors, reducing eye strain and fatigue in the performance of a difficult seeing task. Such eye strain and fatigue are experienced commonly with cool white fluorescent lamps.

 

Old Eyes and Young: Old eyes require a greater quantity of light than young eyes do for the same task. A person needs about 25% greater quantity of light at 50 years of age than one did at 25 years of age; that's about 1% more light per year. Age and the difficulty of the task must be factored into the lighting practice for a particular space. Minimum glare should be the first consideration in the selection and placement of lamps and fixtures. Glare is any brightness that causes discomfort or interference with vision, resulting in ey-strain and fatigue.

 

Fluorescent lamps, which produce most of their light output in the yellow-green spectral region are high brightness sources (such as cool white and warm white). Vita-Lite fluorescent lamps are ideal for reduced glare in interior lighting, aiding in "seeability". Incandescent and high intensity discharge (HID) lamps are high brightness sources and require special fixture design for glare reduction and control.

 

The Eye's Seeing Mechanism: The eye is frequently compared to a camera. But the eye is a living organ and is much more complex than the most sophisticated camera. Both camera and eye have a lens, which focuses an inverted image on a light-sensitive surface. In the camera, the light-sensitive surface is the film. In the eye, it is the retina with its light-receiving cells called rods and cones.

 

The rods function in black-and-white vision like black-and-white film; the cones are for color vision like color film. If the light source produces the natural color rendering of objects as Vita-Lite does, then colors captured in the picture on both the film and on the retina are true and the same. This is "seeability".

 

The eyelid has a function similar to the camera shutter. The iris is the diaphragm, which opens and closes the pupil to control the amount of light entering the eye. These comparisons are shown in the diagram of the eye and camera below.

 

When all parts of the eye are working together as they should, and the lighting is right, the smallest detail and color difference can be seen. This is seeability, the visual acuity to distinguish detail and perform seeing tasks with ease, comfort, speed and accuracy -- and without strain and fatigue. The studies made at Cornell University proved that visual acuity was actually improved, and fatigue was decreased, for students studying under Vita-Lite compared to those studying under cool white. This is "seeability".

 

Comparison of the human eye and a camera

 

 

About the Footcandle: The footcandle was established as a measurement of light quantity by the International Commission on Illumination (CIE) in 1924, when the only electric light source was the incandescent lamp. The footcandle appeared to be an adequate unit at that time, but with the advent of modern light sources, it has become inadequate for determining "seeability". In spite of its inadequacy, the footcandle has persisted in the industry as a dominant factor in the measurement of illumination. Yet, such aspects of light as color temperature, spectral power distribution and color rendering are just as important, or perhaps even more important, in describing seeability in the light from modern light sources.

 

The footcandle has its basis on the standard CIE Spectral Luminous Efficiency Curve for the light adapted eye for daylight vision (cone vision). The curve shown below is based upon the visual brightness of light sources at each wavelength. It can be seen that the "brightest" color to the eye is in the yellowgreen at 555 nanometers, with brightness decreasing at the shorter blue and longer red wavelengths. The Spectral Luminous Efficiency Curve (or, more correctly, the Spectral Brightness Efficiency Curve) is the basis for terms used in illumination and illuminating engineering. For example: the lumen, the international unit of luminous flux, is based on this curve, and the lumen output of a lamp is determined by the amount of light (power) which falls under this curve. Flux is the flow of light from a source.

 

Diagram 1 - CIE spectral luminous (brightness) efficiency curve

Diagram 2 - A light source in the center of a sphere of one foot radius produces luminous intensity of one candle (candle), producing illumination of one footcandle (one lumen per square foot).

 

 

 

Seeing Is Believing: All lamps used for visual purposes are measured for total luminous flux expressed in lumens, a measurement found in our Schedule of Prices and Specifications under the column, "Approximate Initial Lumens". For example: the lumens listed for a 20-watt T12-24" Vita-Lite are 870, nearly the same as the 75-watt A-21 or AT-19 incandescent. Because the lumen is limited in the description of fluorescent lamps, we include the Color Temperature and Color Rendering Index to define lamps more accurately for seeability. Vita-Lite literature contains the spectral power distribution, which is most descriptive of the light output of a light source in terms of its effects on seeability.

 

Proof that the footcandle does not adequately define light for seeability is documented in research showing that light from high-color-rendering lamps such as Vita-Lite provided equal seeability at 25% to 80% of the footcandle level of a standard high-lumen-output lamp such as cool white. This is why Vita-Lite lamps can be substituted for cool white lamps of higher footcandles and still improve seeability. Any task that is adequately lighted with cool white will have improved seeability under Vita-lite. Vita-Lite is then the ideal replacement for cool white or any other fluorescent lamps, wherever they are used.

 

In calculating the number of lamps required for task and area lighting (using the recommended illumination levels in the Illuminating Engineering Society Lighting Handbook Applications Volume, 1981) simply use lamp data for cool white lamps and then substitute them with Vita-Lite lamps in the lighting system. This technique provides a lighting system with the same installed wattage and with enhanced seeability. The difference in seeability between cool white lamps and the Vita-Lite lamps is readily apparent.