What is a Neutral Density?

Monday, April 2, 2007 || Apollo

As you thumb through your Apollo Gel book, you have probably stumbled upon a neutral density filter (AP2310, 2320, 2330) or two and wondered what does this do and why would I need it. The neutral density filter is a basic light fixture or lamp dimmer that can be used in several different ways. It was originally created for the film industry where the use of dimming for the lighting of a scene was not always practical. Some film lights are discharge type fixtures that require ballasts and cannot be dimmed. The neutral density (usually called an “ND”) can be used to “dim down” the fixture to match the other fixtures or to only add a subtle fill to part of a scene. ND’s block light output without affecting the color temperature of the fixture.

In architectural lighting, an ND can be used to drop down the output of harsh fluorescents or discharge lamps to match other sources. In today’s lighting world of brighter is better, the ND is a good tool to remember.

So you are probably thinking, “How does this help me, the theatrical lighting designer?” Well, with the bevy of various lamp wattages and fixture efficiencies out there, an ND can be a godsend. Let’s say that you only have an ETC Source Four with a 750 watt HPL lamp and an Altman 360Q with a 500 watt lamp to project 2 gobos on your cyc and only 1 dimmer left. There will be a big intensity difference between the two fixtures. With the use of an ND, the Source Four can be dimmed to match the 360Q. Cool huh?

So you may not need 100 sheets of the neutral density filters in your gel file but a few of each density will come in handy at around 11:00pm when you are trying to match those fixtures before the director starts screaming.