Photometrics
Each fixture in the film industry will have some sort of photometric information provided along with the light. On these data sheets you'll usually see information about how much light is given off by the fixture at a given distance. This could be in a wide variety of different methods, but we will always try and convert them back to foot candles.
A foot-candle can be defined as the illuminance of one lumen on a one-square foot surface with a uniform distribution.
Photometric App
The Photometric App will do most of the difficult math for you. You are able to type in your camera settings and how far away you will be from the light, then it will tell you what lights will give you proper exposure from that distance and with those settings.
Photometrics with Multiple Source Lights
The inverse square law begins to break down when you have multiple sources in one light and you are very close to that fixture. This could be something as simple as a tube with multiple emitters, or it could be an array of par 64's.
You will need to be at least as far away as the diameter of the sources emitting surface. For example if you are using a single 8' tube, you will need to be 8' away.
Collecting Your Own Data
To find the foot-candles for a given light you'll need:
- The Fixture
Use the fixture that you will take with you on set, if possible.
- Tape measure
- Incident Light Meter
You will need one that reads foot candles.
How to take the incident reading:
- Use a tape measure to find 10' away from the light.
- Take an incident reading with the dome in.
- Type in this number into the photometrics app and all the different distances will be calculated for you.
To Determine Beam Angle
- Will need to update this as beam spread is a specific amount the light diminishes from the center. It may be 10% less than the center or something like that:
- at a certain distance from the fixture you can figure out beam spread by walking left and right in the same plane from that certain distance from the fixture. The dramatic dip in Fc will be the edge of the beam. (Beam angle has a pretty specific definition, need to update this on how to actually figure it out.)
- Find the edge of both sides of the beam and measure the distance between them on the same plane away from the fixture
Calculating Stop Loss of Filtration
Many times when lighting filtration is used to create a certain look. Diffusion and color correction gel are the most common. In this link is a table of types of filteration and the stop loss that can be used in pre pro lighting design.