Shedding light… on light


Tom Draper, manager of marketing for lighting supplier Grote Industries, believes fleet owners would be advised to learn a lot more about “lumens, light and lamps” if they want to ensure they get the most bang for their buck when spec’ing truck lighting products.

“There are a lot of work lamps, headlamps, interior lamps and auxiliary lamps out there,” Draper told me, “and strangly enough, their manufacturers all try to describe them in the most favorable light.  But when we manufacturers do, we sometimes use terms that are ambiguous or use inappropriate terminology for a given application.”

That being said, Draper points out that for starters, no single term of measurement is sufficient to accurately describe the illumination characteristics of a specific lamp – even though that term may of itself be completely accurate.

“So if someone makes a statement that ‘this lamp is just what you need-- it puts out 2500 lumens,’ the point may be technically correct,” Draper explains, “but he may be leaving you in the dark when it comes to the various characteristics the lamp has.”

Instead, he suggests buyers ask sellers these key questions:

·         What purpose is the lamp designed for?

·         Does the lamp meet government regulations?

·         Will the lamp fit my vehicle and application?

·         Will the beam pattern of that lamp correctly illuminate the right areas for my application?

·         How long will the lamp last?

·         How much power will the lamp use?

Draper also says that it is important for fleet managers to understand some specifci terms related to lighting and its performance and lays them out for us:


Flux is the total output of a lamp within a period of time.  For example, we can measure how much water comes out of a hose by pouring it into a bucket and measuring how much time it takes to fill the bucket.  If it takes five seconds to fill up a five-gallon bucket, the flux of water from the hose would be one gallon per second.  That stands regardless of where the hose is pointed.  For water, flux is determined by the diameter of the hose and the water pressure within the hose.

We can also measure the total flux of a light source.  It is called the luminous flux, and is measured in lumens.  A simple light bulb throws light in all directions except the angles blocked by its stem and its cap.  We measure the luminous flux of a bulb by putting it inside an Integrating Sphere, which collects all the light from the bulb regardless of direction and compares it to a common standard. The same is done for work lamps, headlights and so forth.  An Integrating Sphere also captures all the lumens these lamps emit. It’s just that lenses and/or reflectors determine the pattern a lamp is throwing its lumens.

Another way to measure flux is to collect all the light in an Integrating Sphere, and measure the light intensity at each angle around the bulb and then add them all up.  This is an older way of thinking about it.  When we measure the flux this way, the unit is called Mean Spherical Candlepower (MSCP).  You can convert MSCP to lumens by multiplying by 4 pi (the surface area of a sphere) or 4 x 3.14159, so 1 MSCP = 4π lumens.


Another measurement we can make with our water hose is the intensity of the water spray.  When we put our thumb over the end of the hose, the same amount of water comes out, but it sprays out much harder because we are forcing it through a smaller hole.  We could also spray the water out of the hose through a fan.  In some parts of that fan, the water might be spraying very hard, while in other parts it might be softer.

When we measure the light coming out of a lamp in a specific direction, we are measuring the luminous intensity.  The unit of measurement for luminous intensity is the candela.  Just as in our hose analogy, the intensity in candela must be measured at a specific angle and, just like our water fan, intensity can be different at different angles depending on the design of the lamp.


Now imagine watering your garden.  If you want to know how much water ends up on each part of your garden, you could divide the garden up into squares and measure how much water ends up in each square.   We can measure light hitting the ground in a similar way.  If we divide the ground up into 1-meter squares, the unit used is the lux.  If we divide it into 1-foot squares, the unit is footcandles.  In general parlance, the term brightness could be used for illuminance; however the concept of brightness does not lend itself to brightness.


The electrical power used by a lamp is measured in watts, but only a part of that electrical power gets turned into light.  The rest is wasted as heat. Watts = volts X amps.  To make a lamp that is using obsolete bulb technology throw out more lumens, we would have to use more watts, which in turn would create more heat, which would tend to decrease the light expectancy of the lamp and increase the amount of electricity necessary to power the lamp.  One of the reasons LED (light-emitting diode) lamps are replacing older lamp technologies is that LEDs are much more efficient than bulbs, so they can generate the same amount of light with fewer watts of power and last many times longer than older technologies.

Getting back to his shoppers’ list of questions, Draper addresses what purpose is the map designed for and does its beam pattern illuminate the right areas.

“Lenses focus the lumens of a specific lamp to direct a specific amount of candellas in a certain pattern, which can be characterized as the lux patern, to meet the requirements of a certain illumination task,” he explains. “Thus, a spotlight does not produce the light pattern a flood or wide flood does.  Therefore a buyer needs to know the lux pattern of a lamp to know if it suits the job he needs it to do.  It also follows that by themselves, a lumen number or candella number does not give the information necessary to make a correct purchase.  In other words, a low lumen spotlight might be a better purchase than a more powerful flood because it’s lux pattern may put more light straight ahead than the more powerful flood light, which is wasting lumens by pointing them in the wrong direction.”

As for how long will the lamp last and how much power will it use, Draper turns to the issue of light quality.  “Color temperature determines the impact on the user of the light his lamps are throwing,” he points out. “It was set up on the Kelvin scale.   Color Temperature is a value describing the color a specific lamp emits.  Lower numbers are considered ‘warm,’ and are good for creating a comfortable mood, such as in a restaurant or a living room..  Higher numbers, leaning towards the blue spectrum are called ‘cool,’ and are best for spotting color or shape contrast. In the middle is ‘white’ light.

“Correctly selected LED lamps, such as the ones we use, operate at about 6,500 Kelvin, with the resulting color combination being most like daylight and best for seeing details,” he continues. “They also allow most of us to do better work over longer periods of time without experiencing eye fatigue.  Generally speaking, halogen lamps have a color temperature around 3500 Kelvin and are ‘warmer’ than LED lamps, thus not as suitable for preventing eye strain in work crews.”

Draper says that therefore, over time, LED lamps have a “more positive impact” on crew members who are using them than do halogen lamps.  “And since they last longer and use less power, LED technology is becoming the go-to technology when you can’t afford a lamp failure degrading your CSA [Compliance Safety Accountability] score and/or have a lot of work to do,” he adds. “That’s why certain LED white-light lamps are often called LED work lamps.”

So be wise and stay out of the dark and keep Draper’s three Ls-- lumens, light and lamps— in sight along with his key questions the next time your fleet is considering a lighting purchase.


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