Understanding Beam Spread in Lighting
Whether you're setting up stage lighting, designing a photography studio, or planning architectural illumination, understanding beam spread is crucial. Beam spread tells you how wide a light beam becomes when it travels a certain distance, helping you determine coverage areas and plan your lighting layout effectively.
What is Beam Spread?
Beam spread is the diameter of the circular light pattern that a fixture creates on a surface at a given distance. It's determined by two factors: the distance from the light source to the surface and the beam angle of the fixture. A wider beam angle or greater distance results in a larger beam spread.
How to Calculate Beam Spread
The beam spread formula uses basic trigonometry to determine how the light expands as it travels:
[\text{Beam Spread} = 2 \times \text{Distance} \times \tan\left(\frac{\text{Angle}}{2}\right)]
Where:
- Beam Spread is the diameter of the light coverage (in inches or centimeters)
- Distance is the distance from light source to surface
- Angle is the beam angle (in radians for the formula, but commonly specified in degrees)
Note on Angle Units:
The tangent function requires the angle to be in radians. If your beam angle is specified in degrees (as most lighting fixtures are), you need to convert it first:
[\text{Angle in radians} = \text{Angle in degrees} \times \frac{\pi}{180}]
Calculation Example
Let's say you have a spotlight positioned 100 inches from a wall with a beam angle of 0.5 radians (approximately 28.6 degrees).
Using our formula:
[\text{Beam Spread} = 2 \times 100 \times \tan\left(\frac{0.5}{2}\right)]
[\text{Beam Spread} = 200 \times \tan(0.25)]
[\text{Beam Spread} = 200 \times 0.2553]
[\text{Beam Spread} \approx 51.07 \text{ inches}]
Where:
- Beam Spread is approximately 51.07 inches
- Distance is 100 inches
- Angle is 0.5 radians (or about 28.6 degrees)
- tan(0.25) equals approximately 0.2553
Practical Applications
Understanding beam spread helps you:
- Stage Lighting: Ensure performers are properly lit without excessive spill
- Photography: Position lights to achieve desired coverage and avoid hotspots
- Architectural Lighting: Calculate how many fixtures you need to illuminate a space
- Display Lighting: Properly highlight products or artwork with appropriate coverage
By knowing the beam spread at various distances, you can position your lights optimally and select fixtures with appropriate beam angles for your specific needs.