Exposure time

[HoloM]

Hi,
I use vrp-m, data sheet says 70uJ/cm ^2 for the maximum diffraction efficiency at density 2.

For measuring I use a OPT202 from burr brown, with the internal 1Mohm resistor. Data sheet says about 0,27v/uW at 532nm.
It is related to an area of 2,29mm x 2,29 mm.

My voltmeter measures 10mV

So I calculate:

t=70uJ*0,27V*2,29mm*2,29mm/(10mV*cm^2*uW)=100s

Can anyone confirm this?

What about the angle of the photodiode? Do I measure parallel to the film or perpendicular to the beam direction?

[Din]

I think you may be out by a factor of 10.

0.27V = 1uW => 1V = 1/0.27 uW = 100/27 uW = > 1 mV = (100/27)/1000 uW= 1/270 uW = 0.0037 uW. You aperture is 2.29mm x 2.29mm = 0.229 x 0.229 cm^2 = 0.05 cm^2. Therefore, power at plate is 0.0037/0.05 uW/cm^2 = 0.074 uW/cm^2. To get ~70 uJ/cm^2, you'd need 1000 seconds of exposure.

What about the angle of the photodiode? Do I measure parallel to the film or perpendicular to the beam direction?

Perpendicular to the beam. It's the flux hitting the photodiode at normal incidence.

[HoloM]

Am I really? Your calculation doesn't take my measurement of 10 mV at the voltmeter into account, I think. You take 1mv as reference?! That's the missing factor of 10?
For measuring beam ratio for white light reflection copy, I measure perpendicular to the reference beam, and parallel to the film for object beam?

[Din]

Am I really? Your calculation doesn't take my measurement of 10 mV at the voltmeter into account, I think. You take 1mv as reference?! That's the missing factor of 10?

Oops! Sorry, you're right. I used 1 mV, not 10 mV.

For measuring beam ratio for white light reflection copy, I measure perpendicular to the reference beam, and parallel to the film for object beam?

Yes, perpendicular to the reference beam. Also, perpendicular to the object beam, whatever that means. Assuming that the reference beam is collimated, you're hitting the diode with a flat wavefront. This means that all the energy of the beam is being dumped into the diode. If the "direction" of the beam energy (the wave vector) hitting the diode is at an angle, then the wavevector that carries the energy has a component perpendicular to the diode and one parallel to the diode. It's the perpendicular component that's being measured. However, in the case of the object beam, this is a diverging source, which makes it difficult to determine the perpendicular component of the energy carried by the wave.

[HoloM]

Hm. Ok so I exposed 100 sec, and it was far too less, to get a reasonable density with three minutes development.
With 360 secs, I got a better density, but still not d=2 in three minutes.

So,what's wrong with my calculation? It seems that 1000s would heave been the better choice!?
Have I misunderstood any of the data sheets?

i used fresh cwc2.

[holomaker]

Every set up is unique and so many variables, I always bracket my exposures and ratios when doing transmission masters. I suggest you do a quick bracket to test your density out , no need to settle as u are only concerned with density to start .... Small test pieces will go far at making your holograms bright! Here's a snap shot from (homemade components) of a simple mask I adhere to my film with a couple clips... Once you get satisfactory results take your readings , and this will be a good starting point for your next hologram.....

[Ed Wesly]

Here's another way of making test strips: http://edweslystudio.com/Pedagogy/TS/Te ... meset.html

[BobH]

Specification of an exposure energy from a film maker must also specify the developer they used. If you don't do the same processing, their numbers are meaningless. I always used Development in D-19 mixed at full strength for one minute to calibrate materials for myself, mixing it a liter at a time from formula, not the bag. I never evaluated density after development, thinking it cumbersome, impractical and inconsistent.

[Din]

There's a great deal more to exposure than a simple number, or even a simple number and a processing scheme. Holography is still largely a process involving intuition and understanding. It's not like electronics, where you put together a bunch of components, throw a switch and an led lights up or something. Neither is it like software, where you write some code and, if written properly, you run it and an led lights up or something. Holography requires time and patience. You've got to actually shoot a whole bunch of holograms and get a "feel' for the process.

Any book/paper/internet whatever-you-call-them that says "expose to 100 uJ and develop to OD 2.0" should be seen as a guide, not an algorithm.

Holography tends to attract (or, at least, tended to attract) people who think in novel and unconventional ways (I hate phrases like "think out of the box"! People who "think out of the box" simply think inside a larger box.). I think one reason may be that, while holography is a process, it's a process that requires intuition; it's not algorithmic.

[lobaz]

Why perpendicular to the object or the reference beam, Din?
Imagine e.g. a geometry of the Mach-Zehnder interferometer: the beam is split 50/50 to object/reference (at an angle 90 degrees), reflected by two mirrors, spread by the same optics, and instead of the final beam splitter, there is a plate. If the plate is facing one beam, the other illuminates it at an angle 90 degrees, i.e. in fact does not illuminate it at all. Rotate the plate, and the power from the first arm gets smaller, while power from the second arm gets higher.
If I measure the power with the probe perpendicular to the beams, I would get 1:1 ratio regardless of the plate orientation.