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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