Measuring Light Output

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Below is glossary of key terms used when discussing light output.

Glossary

Luminous Flux

A general term for the visible light energy emitted by a source, measured in Lumens.

Lumens-

The standard unit of measurement for Luminous Flux (light output). The quantity of light energy emitted by a source within a defined beam. This is different than the brightness of a light that we measure from a position in space. Lumens are are measured using an Integrating Sphere to capture total output energy. 1 Lumen = 1 Candela-Steradian.

LUX-

Unit of brightness or illuminance of a light that we measure at a given position in space. 1 LUX = 1 Lumen per square meter.

Note: The spread of the light beam has a direct impact on the light intensity that we measure. For example: A light with a total flux of 1000 Lumens, spread uniformly over an area of 1 square meter, will have a measured brightness of 1000 LUX. However, the same 1000 Lumens spread over an area of 10 square meters will produce a dimmer brightness of only 100 LUX. The total output energy of the light remains the same in this example, but the spread has changed its brightness at the position we are measuring from. This is import in understanding how focus/flood works when using a parabolic reflector or a fresnel lens, and how it affects the light intensity. The total energy may remain the same, but the beam angle will determine how bright it measures at any given position. The wider the beam, the less the measured brightness of the light. The more narrow the beam, the greater the brightness.

F/Stop-

Watts-

The electrical energy needed to power a given light fixture. This is not brightness, but rather how much energy the light consumes. The wattage may provide a general indication of the brightness of the light, but output brightness will always be dependent on the spread of the beam and the type of modifier being used.

Candela-

Unit of brightness equal to roughly that of a common wax candle.

Foot Candel-

Unit of brightness equal to 1 Lumen per Square Foot.

Omni-Directional Light

Omni-directional light radiates in all directions from the source. Think of the emitted light as a sphere that continues to grow in area. In this case, as the distance from the source increases by 2x, the area of the sphere of light grows by 4x (see Image 1/2 above). As a result, the amount of light rays hitting a given area or subject decreases to 1/4 (the inverse square of 2, which is 2 stops of light less). If the subject is placed too far away from the source, this can result in very "dull" looking lighting, because the density of light on the subject decreases so drastically. However, this can also be used to your advantage: if the light is placed close to the subject, it can be used to create very dramatic and appealing images, since the the light falloff is so extreme. Generally speaking, the more a light is diffused or omni-directional - like a softbox, white umbrella, or other diffused source - placing the light closer to the subject will give more appealing results. Each modifier is slightly different, however, so it is important to test where the "sweet spot" is for you.

Focused light

A parabolic reflector is a light collimator - that is, it focuses the light into a straight and narrow beam. In this case, the entire output of the source is directed very efficiently into parallel rays with minimum spread. Therefore, the density of the light rays - and the intensity of the light - remains much more consistent as the light travels (see Image 2/2 above). For this reason, the Inverse Square Law does not apply to focused light. As a result, when using a parabolic reflector in a focused position, the subject can be placed much farther away from the light without loosing the vibrant quality of the light. We usually refer to light like this as having a long "throw". Using the Focusing Mount, as you move the light source away from the Focus Position inside the reflector, the light starts to spread out and behave more like an omni-directional source - gradually, the inverse square law applies when the light is fully flooded. Remember, the more the light spreads out, the closer it should be placed to the subject. Overall, this is one of the reasons why a Parabolic Reflector is such a versatile light shaper - because it can act as both a focused light, and a diffused light, and everything in between!

In Practice

In practice, there is always some widening and softening of the light beam even when using a focused parabolic reflector - it is not as precise as a laser (although the concept is the same). This can actually be considered a benefit in many ways, because it leads to a light that has some inherent softening, which is favorable in photography lighting. Because of this, the light will have some degree of falloff and spread, but not as much as an omni-directional source.

Summary

Forget everything you've learned about the Inverse Square Law from photography blogs - these can be very misleading.
Light doesn't mysteriously "die out" as it travels - it either spreads out, or it is absorbed or reflected when it hits an object.
If a beam of light spreads out, the density of light rays decreases in a given area (what we interpret as the light loosing its intensity).
The Inverse Square Law is a property of spherical geometry, not light! Omni-directional light fills an every-expanding sphere of coverage as it travels away from the source. As the distance from the source increases by 2x, the area of this sphere increases by 4x, and therefore the density of light rays in a given area decreases to 1/4 (the inverse square of 2).
Focused Parabolic Lighting emits light in collimated (parallel) light rays, therefore the Inverse Square Law does NOT apply.
The more focused a light source, the further the "throw" of the light (the light can be placed further away from the subject).
The more diffused the light source, the more quickly it looses its measured intensity, and the closer you want to place the light to your subject.

Further Info

After reading and understanding the above, you can now start to understand why energy collimation is used so often in many scientific fields (satellite dishes, for example) - to transmit or receive energy to and from very far distances without loosing its intensity. This applies to light waves, radio waves, radar, sonar, etc. Another example: a lighthouse uses a focused Fresnel lens to "throw" a light many miles - this is only possible because the light beam is collimated, and therefore does not loose its intensity as it travels.

Click here for more information on the Inverse Square Law, or just Google it!

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