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What is the difference between Lumens, Lux, Watt, Lumen per watt and color temperature?

When buying a new dive light, it is easy to get confused about the different terms that are used to describe the most important parameters of the light.

LUMENS

Lumen is a unit of light, which is also known as Luminous flux. Lumens (lm) are a measurement unit, which tells what the total amount of light emitted from a dive light. You can roughly say that the more Lumens the brighter the light.
When we specify our dive lights, we use Lumens to see the total amount of light output. But Lumens will only show us a part of the picture. Producing a perfect beam shape does not reveal enough information to show us how the light output is created. For this we need a lux meter.
To measure the total amount of lumens, an integrating sphere is necessary. An integrating sphere (also known as an Ulbricht sphere) is a hollow, spherical chamber coated internally with a high reflectance coating that exhibits diffuse reflectance. Spheres are used as directionally-insensitive collectors of light when combined with photodetectors. An internally illuminated integrating sphere emits a field of spatially and angularly uniform luminance or radiance which is perfect for testing led lights.

LUX

Lux (lx) is the SI unit of illuminance and luminous emittance, measuring luminous flux per unit area. One lux is equal to one lumen per square metre. In photometry, this is used as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface.

Lux is much easier to measure than Lumen. Lux can be measured with a hand held device.
If the light output is focused on a small area (narrow beam), we see this as very bright light. If the light output is focused on a greater area (wide beam), we experience this as a weaker light.

WATT

Watt (W) is a derived unit of power in the SI units, defined as 1 joule per second and can be used to quantify the rate of energy transfer. It shows how much energy the product consumes, not how much light output (lumens) it provides. For this reason you should not only look after the amount of watt consumed, when select a dive light. It will just tell you how quickly it will drain your battery, and not how much light it produces.

LUMINOUS efficacy

Luminous efficiency is a measure of how well a light source produces visible light. It is the ratio of luminous flux to power, measured in lumens per watt  (lm/W) in SI unit.

COLOR TEMPERATURE

The color temperature of a light source is the temperature of an ideal black-body radiator that radiates light of a color comparable to that of the light source. Color temperature is conventionally expressed in kelvins (K) is the SI unit.

Color temperatures over 5000 K are called “cool colors” (bluish white), while lower color temperatures (2700–3000 K) are called “warm colors” (yellowish white through red). “Warm” in this context is an analogy to radiated heat flux of traditional incandescent lighting rather than temperature. The spectral peak of warm-coloured light is closer to infrared, and most natural warm-coloured light sources emit significant infrared radiation.

The Sun closely approximates a black-body radiator. The effective temperature, defined by the total radiative power per square unit, is about 5780 K. The color temperature of sunlight above the atmosphere is about 5900 K.

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Shape of a Light Distribution Curve

In illumination engineering it is very important to see the total shape of the light distribution curve. A light distribution curve is a 2D- or polar diagram -characterization of the performance and it tells for an experienced eye what in detail to expect of the component, e.g., how narrow the light distribution is, are there any discontinuation points to be expected (shadows) or what the relative intensity is in HV 0 degree vs. 30 degrees.

A Full Width Half Maximum (FWHM) angle has been defined, in relative terms, for a symmetrical optics with its maximum intensity in the middle of its light distribution (horizontal and vertical 0 degree), to be the angle, where the intensity of illumination has dropped to 50% from its maximum peak value. Furthermore, many advanced companies define a further so-called 10% value, which is the angle, where the intensity of the illumination has dropped to 10% of its maximum peak value. This is a very useful parameter, e.g., when specifying optical components with an extremely narrow light distribution. The closer the 10% value is to the FWHM value the more light is really focused in the important narrow beam and the less stray light you have outside of the main beam. Now, one may wonder, why to use two values for a component, FWHM and 10% value, why is not FWHM itself sufficient? The reason is that FWHM value is not unambiguous, and it can even be misused to mislead a person specifying his system.

Let’s take a simple example with imaginative lenses A and B.

FWHM value does not give unambiguous information of an optical component.
These two lenses have the same FWHM value, but they perform in a very different way

 

Lens A is a lens with relatively bad optical efficiency and additionally, a proportionally big share of light falls outside of the centre beam area, i.e., its 10% value is a wide-angle value. Due to the shape of its light distribution curve – a flat curve with no really high peak in the middle, but more or less a “hill” type of a shape – it still has a FWHM value of +/- 5 degrees.

The other lens, Lens B, is a lens with high optical efficiency, with very concentrated beam and a narrow-angle 10% value. Its curve shape reminds of a Himalayan mountain instead the hill for lens B. The surprising fact is that this lens has the same FWHM value of +/- 5 degrees, as lens A. How can it be possible? Putting the absolute (not relative) curves of these 2 lenses on top of each other in the same diagram, shows that lens B gives 5x the light than lens A, but still the FWHM values are identical! The conclusion of this simple example is that different lenses cannot be compared against each other just using FWHM values. FWHM does not give the answer to the question how much absolute light is distributed in the specified angle or area. More facts are needed, 10% value already gives a good hint of how an optical component performs.

 

Source: Tomi Kuntze, President, LEDIL Oy