Lighting Metrics ~ Watts, Lumens and Efficacy:
Choosing the right "lighting system" (lamp & luminaire) for a specific application can seem complicated, especially since Cruisers often lack the technical knowledge needed to make fully informed decisions. Many different elements of lamp performance must be considered. A basic understanding of the science and technology of light may help to illuminate the decision-making process.
I hope this helps, a little:
Many people think that a higher wattage lamp will always produce more light than a lower wattage one. This confuses light output, which is measured in lumens, with the electric
power a lamp uses, which is measured in watts. In fact, a 20W compact fluorescent lamp can produce just as much usable light as a 75W incandescent lamp.
The most common way to express the “efficacy” (energy efficiency) of a light source is
the LPW ratio. This is the number of lumens it produces to each watt of power it consumes. The LPW is a very important metric on energy-sensitive boats.
Fluorescent and HID lamps require ballasts for operation, so the lamp(s) and ballast work together as a “system.” The efficacies of these systems must take into account the actual lumen output of the lamp(s), as well as the power drawn by both the lamp(s) and the ballast. Lamp and ballast combinations must be specifically designed to work together to provide high system efficacy.
Service Life and Lumen Maintenance
The average rated service
life of a lamp is based on the point in time when 50 percent of a large sample of those lamps will fail, or “burn out.” Many of today’s most sophisticated lamps offer an extremely long service
life as well as exceptionally high color rendering. Lamps may cost more to purchase
, but the additional expense can be offset by reduced energy costs and less frequent replacement. The light output of all lamps will deteriorate gradually over time. The rate of this deterioration ( lumen maintenance) varies from lamp type to lamp type. It is particularly important to understand lumen maintenance
and service life when selecting a lamp for a hard-to reach fixture (anchor light) where replacement is difficult.
Footcandles and Candlepower
Two important measurements of light are frequently confused - Footcandles and Candlepower.
Footcandles measure the light that falls on a surface (illuminance) in lumens per square foot.
Candlepower, however, measures the intensity of a light source in a specific direction. Candlepower measurements are expressed in candelas and are independent of any object or surface that is being lit.
There are two systems of measurement commonly used to describe the color properties of a light source: “color temperature,” which expresses the color appearance of the light itself, and “color rendering index” (CRI), which suggests how an object illuminated by that light will appear in relation to its appearance under other common light sources. Both can be extremely valuable in evaluating and specifying light sources, but it is important to understand their limitations.
The Color Temperature
of a light source is a numerical measurement of its color appearance. It is based on the principle that any object will emit light if it is heated to a high enough temperature, and that the color of that light will shift in a predictable manner as the temperature is increased. The system is based on the color changes of a theoretical “blackbody radiator” as it is heated from a cold black to a white hot state. A light source’s color temperature is the temperature, measured in degrees kelvin, expressed in kelvin (K), at which the color of the blackbody would exactly match the color of the light source
It may seem counterintuitive that low color temperature light sources are called “warm” while those with higher temperatures are referred to as “cool.” In fact, these descriptions have nothing to do with the temperature of the blackbody radiator but refer to the way color groups are perceived.
Colors and light sources from the blue end of the spectrum are referred to as cool, and those toward the red/ orange/yellow side of the spectrum are described as warm.
The Color Rendering Index
(CRI) is a system derived from visual experiments. It assesses the impact of different light sources on the perceived color of objects and surfaces. The first step is to determine the color temperature of the light source being rated. Next, each of eight standard color samples is illuminated - first by the light source and then by a light from a blackbody matched to the same color temperature. If none of the samples changes in color appearance, the light source is given a CRI rating of 100. Any changes in color appearance which do occur result in a lower rating. The CRI decreases as the average change in the color appearance of the eight samples increases. In general, a high CRI figure means a light source will render colors well. Any CRI rating of 80 or above is normally considered high and indicates that the source has good color properties.
Point Source and Diffuse Source
When selecting a lamp for a given application it is important to determine whether a point source or a diffuse source is more appropriate. A point source is a lamp or fixture that is relatively small compared to the area it is lighting and has the potential to direct a concentrated beam of light on a specific surface or object. Incandescent, halogen and HID lamps are typically used in point source applications. A diffuse source, on the other hand, is relatively large and spreads light over a wide area. Linear fluorescent lamps are the most common diffuse source lighting.
Beam Angle and Field Angle
The pattern of light delivered by reflector lamps (point source) is often described in terms of beam angle and field angle. Beam angle is the smaller figure, and refers to that portion of the lamp’s beam where the candlepower is greater than 50 percent of the candlepower measured at the center of the beam. The field angle describes the larger area of the beam where the candlepower is greater than 10 percent of the center beam candlepower.
PS: If anyone's interested, I could prepare a summary comparison of various lamp characteristics, and/or a more complete lighting design manual. I used to (20-25 years ago) teach a lighting design course. These would be "lengthy" compositions(s), only worth the (considerable) effort, if anyone's truly interested.