Satco Products HID Lamps Manuel D’Utilisation

HID Lamps Notes

HID lamps

120

Studies on nighttime visibility demonstrate experimentally 

that the human eyes is sensitive to different colors of 

light based on various lighting levels, thus impacting 

the “true” or “effective” lumen output of a lamp. Recent 

research shows that the color of a light source has a 

significant effect on nighttime visibility, which is important 

since most road accidents happen at night. It is also well 

known that the eye responds to color differently based 

on the amount of light available.

While lumens are the standard measurement of light 

output, light is actually defined as energy evaluated by 

the eye. Standard lumen measurements are based on 

the light output response of a person only during high 

light levels (photopic light), typical of daylight and interior 

lighting. Light meters measure photopic light as seen 

by the central region of the eye. When light levels are 

very low, like starlight, the viewing conditions are referred 

to as scotopic. Under these conditions, the eyes visual 

response changes dramatically. Sensitivity to yellow and 

red light is greatly reduced, while response to blue light is 

vastly increased. Since lamp lumen measurements have 

been determined using photopic measurements, the 

lumen values do not accurately measure the effective 

light output as perceived by the human eye. 

Since the value of a lamp’s lumen output is different 

when considering the shifting color sensitivity of the eye 

at low levels, the effective lumens will be different from 

the measurable photopic lumens. As light diminishes 

from photopic to scotopic conditions, the effective 

lumens of yellow sodium light sources are reduced, and 

the effective lumens of the whiter metal halide sources 

increase. This effect is most dramatic for low pressure 

sodium (SOX) lamps. Almost all energy output from SOX 

lamps is yellow, resulting in an artificially high photopic 

lumen output, yet a very low perceived light output in 

low light level applications.

Typical metal halide lamps have strong light output 

in the blue, green and yellow areas, resulting in high 

lumen output at all light levels. The blue light output 

of metal halide is in the high sensitivity region of the 

eye for low light levels. This means that the effective 

lumens actually increase for a metal halide lamp as 

the light level reduces and the eye shifts to a blue/

green peak sensitivity. The ability to detect fine contrast 

is also significantly better under metal halide sources 

than sodium, making it a more effective light source in 

warehouse and retail applications.  Studies have shown 

that metal halide lighting, in some circumstances, can 

be up to six times as effective as HPS. This can make a 

difference in peripheral viewing and dark areas where 
hidden hazards may be present.

The 2005 update to the National Electrical Code (NEC) 
includes two changes that affect the manufacture 
and installation of metal halide luminaires. Both 
changes address methods for preventing risk when 
metal halide lamps are used. 

The purpose of these changes is to prevent possible 
injury and property damage. Metal halide lamps can 
explode, shooting hot glass out of the luminaire, or 
fixture. In an open luminaire - one that does not have 
a lens - this can result in a fire, personal injury, and 
property damage. Although violent failure is a rare 
occurrence (about three incidents per year), the NEC 
has included these changes to help eliminate the 
hazard. 

In sports facilities, particularly in schools, athletic 
activity can break the outer jacket of a metal 
halide lamp used in an open luminaire. When this 
happens, the arc tube can continue to operate with a 
damaged or missing outer jacket. This leads to reports 
of UV over-exposure, including sunburn and a burning 
sensation around the eyes. The NEC change requires 
complete lamp enclosure in these facilities, which 
will provide mechanical protection to reduce the 
possibility of damage to the outer lamp jacket. 

The NEC change regarding open luminaires is the 
new section 410.73(F)(5) that states, “Metal Halide 
Lamp Containment. Luminaires (fixtures) that use a 
metal halide lamp other than a thick-glass parabolic 
reflector lamp (PAR) shall be provided with a 
containment barrier that encloses the lamp, or shall 
be provided with a physical means that only allows 
the use of a lamp that is Type O.” 

Therefore, to meet the 2005 NEC requirements, 
luminaires that use metal halide lamps must either: 

• Be enclosed to provide a containment barrier. 

•  Use a special lampholder that will only accept an 

ANSI Type O rated metal halide lamp. 

 

The NEC change regarding open luminaires is the 
new section 410.4(E) that states, “Luminaires (fixtures) 
subject to physical damage, using a mercury vapor or 
metal halide lamp, installed in playing and spectator 
seating areas of indoor sports, mixed-use, or all-
purpose facilities shall be of the type that protects 
the lamp with a glass or plastic lens. Such luminaires 
(fixtures) shall be permitted to have an additional 
guard.”