Originally Posted by darinp2
There is the part about being able to shut down further, but dual irises where one is between the lens and imaging chip(s) and one is in the lens tend to improve native on/off CR better than a single iris in the lens for the same reduction in white level.
DLPs are different than LCOS, but I'll use a DLP as an example. Using round numbers from what I recall, with the Sharp 12k we could get about 1200:1 and 900 lumens with both irises open. Closing just the lens iris would reduce the white level significantly (more than half IIRC) and result in somewhere around 2000:1 on/off CR. Now closing the other iris didn't reduce the white level much more, but got close to 4000:1 on/off CR at around 300 lumens.
Those irises just had one position each. With the last JVC with dual irises I checked out each step down would close one of the irises a little more, alternating with each step to keep them in sync.
I tried to explain why I thought a dual iris in a DLP would have that kind of effect, after I modified an Optoma H79 to about 9000:1 on/off CR (but not a lot of light). I'll try again.
Let's consider a system where if we consider the cross-section of the lens where an iris would be placed as having a uniform intensity for white that is the size of a dime and a uniform intensity for black that is the size of a quarter and all from scattered light. If we put a round iris after the lamp and a round dimed sized iris for the white in the lens, then we get a certain amount of light through for white and a certain amount for black, resulting in a specific on/off CR. now what happens using ideal assumptions if we cut the iris in the lens in half? Both white and black go down to half (since area goes to half and intensity per area stays the same), leaving the same on/off CR.
However, if we then go cut the lamp iris in half such that it perfectly lines up with the lens iris, what happens? If he light for white is perfectly controlled then the white level stays the same. The white light that is being blocked by the lamp iris was blocked by lens iris before anyway. However, the same is not true for the light for black since that is scattered light and uncontrolled. By blocking half the light for black before it even gets to the imaging chips there is half as much light to scatter in the lens. With this 2nd step the area for the black going through the lens is the same, but the intensity is half as much as without the lens iris. So for this case twice as much on/off CR.
And the irises can be halved again for another doubling of on/off CR for this ideal case. Of course, real world cases don't fit that ideal (the black level through the lens isn't only random light for one), but I believe the factor I described is relevant to on/off CR increasing as dual irises are closed, if they are placed and synced correctly.
I hope that made some sense.
As far as how it relates to the single iris vs dual iris JVCs, the specs are for being able to get some advantage of the dual iris system. If both irises need to be open to get enough light then having both irises doesn't do much good (although the lamp iris may help native at manual position 0, but likely only if it is closing off some light and reducing maximum lumens). If a person could use a middle position with the RS49 they would probably get some advantage to on/off CR with a dual iris model, but I'm not sure how much.