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Why is 'lamp density' important when shopping for lightboxes?

Comparing the light-generating properties among different models of backlit display lightboxes can be difficult when the lightboxes are not in the same room with you. Specifications such as measured brightness, color temperature and type of lamps can be helpful, but don’t overlook the value of lamp density in helping you evaluate competing lightbox models.
WHAT IS 'LAMP DENSITY' AS IT RELATES TO LIGHTBOXES FOR BACKLIT GRAPHIC DISPLAY?
'Lamp density' is basically how many lamps are installed in a given lightbox. More scientifically, lamp density can be measured 3 ways:
distance between lamps, in inches
number of lamps per square area
total number of lamps in a lightbox, given its size
Optimal lamp density vs cabinet depth
WHY DOES LAMP DENSITY IN A BACKLIT DISPLAY LIGHTBOX MATTER?
Lamp density can influence all of these:
Intensity, which is really just brightness per square inch or square foot.
Overall light output (aka ‘brightness’) of the entire lightbox — a larger lightbox will only have more light output than a smaller one if the lamp density is not reduced beyond a certain level.
Diffusion — how uniform is the brightness across the display face of the lightbox?
Color cast or hue, aka ‘color temperature’ – decreasing the lamp density may result in a modified overall color temperature.
Lamp density also influences these other non light related factors:
heat generation
energy consumption
weight of the lightbox
investment — increased lamp density in a lightbox is of course generally correlated with increased value
HOW IS LIGHTBOX LAMP DENSITY DETERMINED IN ENGINEERING?
Architects, general contractors and sign shops often ask, what is the engineering that determines lamp density for a lightbox designed for backlighting graphic artwork? Here are the primary factors:
Chassis depth — the deeper the cabinet, the lower lamp density required to achieve adequate ‘coverage’ (aka diffusion) at the display face. Of course this may compromise brightness, but the reality is – not by much. The more significant downsides of a deeper lightbox frame for many customers are the issues with space planning and/or esthetics.
Lamp orientation — lamps can be configured to directly backlight the display face from behind; or indirectly via a “light guide plate” which scatters the light generated by a single linear light source (gas tube lamp or strip of LED’s) across a two-dimensional backlit face.
Focal dispersion — the wider the focal angle of the lamps, the fewer lamps needed to adequately illuminate the display face.
Artwork — if the type of graphic artwork is known ahead of time, the lamp density can be optimized for it. For example, a solid light-colored background will be less forgiving of low lamp density than a busy, multi-colored background.
Economy — of course, Lightbox Engineering always looks for the best combination of performance and economy – how can we get the best backlit performance for the most efficient price point?
Energy efficiency — reducing lamp density as much as possible without sacrificing performance will of course save energy and at the same type generate less heat.
WHAT LAMP DENSITY SHOULD I LOOK FOR IN A BACKLIT DISPLAY LIGHTBOX?
Here are some more specific questions (with answers) to help you answer this bigger question:
How large is your lightbox going to be? For lightboxes less than 4-5 feet in the smaller dimension, consider “edgelit” lightboxes, for their improved diffusion-to-lamp-density ratio and low profile frames. For larger lightboxes, look for premium edgelit LED and/or direct-backlit fluorescent models with lamp spacing of 7” or less, unless the application includes low-demand artwork and/or the desire for a less-bright backlighting system.
How bright does your lightbox need to be? Almost all lightboxes on the market perform within a range of “adequate” brightness for most commercial applications. The brightest-available lightboxes in the general market usually share most of these features:
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direct-backlit (as compared to edgelit or indirect)
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LED-powered (as compared to fluorescent etc.)
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high-density lamps, such as 1.5” spacing or less
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low-profile chassis models, such as 3” or less total frame depth
Low Density Fluorescent Bulbs
If a lower-than-average-brightness lightbox model is desired, shop for a combination of at least some of these features:
fluorescent lamps
low lamp density, such as 7” spacing or more
deeper chassis depth
'warmer' (lower) color temperature lighting, in the 3000° to 4200° Kelvin range
an added "dimmable" option
What type of artwork are you going to display in your new lightbox? If the artwork has a solid and/or light background, then look for lamp densities of 6” spacing or less. If your artwork is multi-colored and/or has large black or very dark areas, a lower lamp density can still perform adequately in terms of uniform diffusion, as apparent to the eye.
In summary, lamp density is just one of many parameters you can use for comparison as you shop, but it clearly gives you another tangible handle for isolating the ideal product for your backlit graphic display application.
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