Knowledge Base

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

    Grote's Answer

    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.

    General Questions
    Updated Sep 22, 2014

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