Sunglasses for Lights and Windows 😎
ND stands for Neutral Density. The sole purpose of an ND gel is to reduce the amount of light passing through it, acting like a pair of sunglasses. Unlike color correction gels (like CTO and CTB) which are designed to change the color of light, ND gels are engineered to be perfectly neutral gray, ensuring that they do not introduce any color cast. They simply reduce the brightness, or intensity, of a light source.
Why Not Just Dim the Light?
One might ask why you would use an ND gel instead of simply dimming the light. There are two main reasons:
- Color Shift: Many types of lights, especially older tungsten and HMI fixtures, cannot be dimmed without also changing their color temperature. Using an ND gel allows a cinematographer to reduce the brightness while maintaining a stable, consistent color.
- Controlling Ambient Light: The most common use of ND gel is not on lights, but on windows. For a day interior scene, the natural daylight coming through a window is often many times brighter than the artificial lights inside the room. This huge difference in exposure can be impossible for a camera to handle. To solve this, the grip department will carefully apply large sheets of ND gel to the exterior of the windows. This brings the brightness of the outside view down to a level that is balanced with the interior exposure, allowing the camera to see both the actors inside and the view outside clearly.
Measured in ‘Stops’
The strength of an ND gel is measured in ‘stops’ of light reduction. One stop is equivalent to a 50% reduction in light.
- ND.3: Reduces light by 1 stop (50% transmission).
- ND.6: Reduces light by 2 stops (25% transmission).
- ND.9: Reduces light by 3 stops (12.5% transmission).
By combining these standard densities, a gaffer or grip can achieve a very precise level of light reduction.