In order to design a compliant system we must be able to calculate the ‘brightness’ required to achieve the specified System Contrast Ratio.
Gather the following information:
- Ambient light on screen (L) from all natural and artificial sources. This is realistically your ‘black’ level, as the system cannot achieve lower. Measured in lux.
- Target contrast ratio (C)
- Image area (A), which you can easily derive from the Image Size calculations you have prepared. Measured in m
- Screen gain (Sg).
- If you are using a typical vinyl or fibreglass projection surface, the screen should be unity gain (1.0) but maybe slightly above or below
- Specialist screens – particularly ambient light rejection types will usually offer higher gain but have a smaller field of view
- In all cases – ask the screen manufacturer for data if you are unsure.
- Lens derating factor (Dl). It is often difficult to get accurate lens data so the following assumptions are made:
- A ‘standard’ lens is one with throw ratio around 2.0 (in practice often 1.7-2.3) and which has the least complex light path; i.e.: fewest glass elements. Dl = 1.0
- Short- and long-throw lenses generally contain more glass elements and are expected to lose 20% of the light. Dl = 0.8
- Projector derating value (Dr) is applied so we achieve the required System Contrast Ratio on average across the life of the projector.
- A new lamp has 100% output, and end-of-life is 50% brightness then the calculation point is at 75% brightness. Dr = 0.75
Once you have the requisite information, apply this formula to determine the minimum required projector brightness (Luminous flux, measured in lumens):
- 120” 16:9 (2655 x 1495 = 3.97m2)
- Standard vinyl; gain = 1.0
- Ambient light on screen = 80 lux
- Viewing category is BDM, target contrast ratio = 15:1
- Standard lens
= 6352 lumens