Engineering Art Hernandez

Q: Why does my LED flashlight look brighter than a Grote lamp?

A: People often confuse luminous flux with luminous intensity. Luminous flux is a measurement of the total amount of light coming out of a light source. Luminous intensity is a measurement of how much light is aimed in a specific direction. Luminous flux is determined completely by what we use for a light source - how many LEDs, how powerful each LED is, etc. Luminous intensity can be changed by the optics design of the lamp. Lamps designed for different purposes will do different things with the light that affect how much intensity is measured in any one particular direction.

To make things even more difficult, the light coming from the lamps often shines onto a surface, and we look at the spot on the table or floor and compare how bright it appears. The amount of light shining on a particular surface is called the illuminance, and the amount of illuminance on a surface depends on the shape of the beam, the distance from the light source, and the angle between the light beam and the surface. Often the math involved in calculating the illuminance on a particular surface requires high-level calculus or a computer or both. However, some rough calculations can be made for simple cases. Some examples follow.

An LED flashlight
When designing a flashlight, the goal is to make a narrow beam so that the light can cast a bright spot a long distance away. Usually some kind of concentrating optics are used to do this. (A reflector and/or a lens to collect all of the light from the LED and direct it at a specific angle.) A typical LED flashlight uses an LED that emits about 2 lumens of luminous flux. Those 2 lumens are all aimed within a cone of about 3 degrees, giving an intensity of about 730 Cd within that very narrow spot. If that flashlight is held up near the roof of a 6 foot tall trailer, it will illuminate a spot 3.8 inches in diameter on the floor. The illuminance within that spot will be about 230 Lux. Of course, outside that spot you won't see anything.

An LED Dome light
When designing a dome light, the goal is to illuminate an entire room, or at least a very wide space within a room. For this reason, the light emitted by the LEDs in the lamp is spread out into almost a complete hemisphere. One typical Grote dome lamp uses 16 LEDs that emit a total flux of about 640 lumens. This light is spread within a complete hemisphere, or a 180 degree cone, giving an intensity of about 65 Cd within that hemisphere. The light from the dome lamp will also illuminate a much larger spot on the floor of the trailer, about 12 feet in diameter instead of mere inches. Within that large spot, the illuminance will measure about 25 Lux.

So the answer is yes, the LED flashlight is brighter. If you want to light up a small circle, get yourself a flashlight. However, if you want to see everything within the trailer, a dome light is a better choice.
 

Q: Can I compare the candlepower of two lamps to determine which one is brightest?

A: All motor vehicle lamps are required to meet federal specifications regulating the brightness of the lamps.  These regulations require that each lamp be designed to meet certain intensity values at various angles.  The words "candlepower" and "brightness" are popular terms for the technical term luminous intensity.  Intensity is measured in units of Candela, and has no meaning when applied to a lamp as a whole.  Intensity can only be measured at specific angles.

The federal regulations for most lamps require that the lamp be tested at 15 to 20 specific angular locations, called test points.  At each of these test points, the lamp is required to have a luminous intensity greater than a certain minimum candela value.  Often, that intensity must also be less than a certain maximum candela value as well.  When evaluating the performance of a lamp, all of these test points must be considered.  Looking at the candela value at a single test point is not sufficient to understand the overall performance of a lamp.

            All Grote lamps are designed to exceed the appropriate federal regulations for their specific functions. 

Q: Can I still drive a vehicle with LED lamps if some of the LEDs go out?

A:  First of all, this issue should come up only rarely.  The reason many vehicle manufacturers are switching to LED lamps is that they last for a very long time.  One reason those manufacturers choose Grote LED lamps is that they trust our company to make well-designed, durable lamps.  While some of the less reputable lighting suppliers may have LEDs out on their lamps, such an occurrence should be very rare for Grote LED lamps.

 In general, any non-functioning LEDs will make an LED lamp no longer pass the legal specifications.  Since we want to reduce costs to a minimum, we typically design lamps with the minimum number of LEDs that will meet the specifications.  Of course, we do allow a margin to account for manufacturing variations, but on any random production lamp, we do not know what particular variations may be there, and so there is no way we could warrant that a particular lamp would be legal with an LED out.

Often, LED lamps will be designed with several zones of optics, with each zone designed to put light into a particular part of the required optical pattern.  Each of these zones may direct the light from just a few LEDs.  Therefore, if any of those LEDs were to stop functioning, one part of the required light pattern will be dark.  In addition, LED lamps are usually wired with small groups of three or four LEDs in series, so that if a single LED in such a string fails, all of them will lose power.  This means that if any LEDs fail, it is very likely that three or four LEDs will stop functioning.  This means that it is very likely that any lamp in which LEDs fail will no longer meet the government specifications.

Regulations:

            NHTSA and Transport Canada are both studying this issue, but neither of them has reached a conclusion at this point.  Arguments can be made that incandescent bulbs dim over time, or get covered with dirt, etc., and they are still considered legal on a vehicle even though they would no longer pass the laboratory tests.  Similarly, an LED lamp with a few non-functional LEDs still gives a visible signal, even though the intensity of that signal may be somewhat lower than when the lamp left the factory.  These arguments are valid, but in the interests of safety, we should stick to the position held above until the government makes a ruling.