Is defined as a function of Intensity and Time: E = I * t. Every light-sensitive material requires a certain quantity of exposure to do its job properly.
The classical photographic analogy is to compare exposure of film (or an electronic sensor, more than likely in this era) to filling a glass with water. A trickle of water coming out of the faucet will take a long time to fill the glass while a wide-open torrent will take a short time. On a camera, the faucet is replaced by the iris, f/stop, aperture, diaphragm, whatever you want to call it, which limits the flow of light through the lens, while the shutter controls the length of time of photon delivery. To go one step further, the sensitivity of the film, the ISO speed rating, can be modeled by the volume of the cup; high sensitivity, requiring less light to do its job, would be represented by a smaller cup, while a less sensitive film with a lower ISO would be represented by a larger cup to be filled.
For a holographic model of exposure, the flow of water would be represented by the power of the laser; little He-Ne’s and pointers would be like garden hoses, and fire-breathing Argons and YAG’s would be like fire truck hoses.
The sensitivity of the holographic recording material would be represented by the height of the receptacle, while the spread of the laser beam would determine the diameter of the receptacle. A beaker a decimeter tall and a decimeter in diameter could represent the photon thirst of a holographic plate that’s of the convenient 6 by 6 cm (2 ½”) size; along the same lines, a container that is 6 decimeter in diameter that is filled to a depth of one decimeter is the volume of photonic liquid that is needed for a 30 by 40 cm plate. Even a small flow would take a reasonable amount of time to fill the former; the latter, a pretty long time.
Scroll down if this spiel becomes overwhelming. When calculating exposure in holography, the amount of light available per unit area comes into play, the unit standardized upon being “per square centimeter”. Lasers’ output powers, being sources of light, are typically measured in Watts or fractions thereof. A reading of light power delivered to the holographic recording plate is generally described in microwatts per square centimeter, µW/cm² for short, and if you’re dangerously lucky you could be measuring milliwatts per square centimeter, mW/cm², for the fire hoses of the high power lasers.
What’s in a Watt? A Watt is a measure of power; a force that is delivered constantly as long as the source is turned on. Intensity in other words. But exposure integrates intensity during a length of time, which physicists define as energy, so another physics unit, the Joule, is used to define exposure doses.
A Joule is a measure of energy; how much power is delivered during a period of time. A Joule is defined as a Watt – second; a 1 Watt light bulb turned on for one second emits a Joule of energy. Simliarly a 10 Watt bulb on for 1/10 second also delvers a Joule of energy. Notice that a Joule is not a Watt per second which means divided by seconds, but multiplied by seconds.
The units for exposure doses for holographic recording materials are microJoules per square centimeter, µJ/cm², ranging from tens of them for the old high speed materials like Agfa 10E75 and Kodak SO-253 to hundreds for Agfa 8E75HD and Slavich PFG-01 to thousands for Slavich PFG-03M and Sphere-S GEO-3, plus photo-resists and dichromated gelatins, or more properly milliJoules per square centimeter, mJ/cm², since there are a 1,000 micros in a milli.
Calculating exposure times can be based on knowing how many µJ/cm2; are required from the manufacturer’s instructions, measuring the power of the light per unit area at the film plane in µW/cm², and dividing the energy reading into the power requirement to get the exposure time.
Example: A holographic film allegedly requires 200 µJ/cm² to do its job. 50 µW/cm² is measured at the film plane. 200 divided by 50 = 4, so a 4 second exposure would be required.
Stop scrolling here for the bottom line! But not everyone has a laser power meter, nor does one trust their power meter if they do have one, or trust the manufacturer’s energy requirements. An exposure test series, similar to what photographers would do in making a print in the darkroom needs to be made to fine tune the exposure to get the optimum result. The intensity part of the exposure is determined by how much the given laser beam’s power is spread over the area to be holographed; the holographer finds the appropriate time at first by trial and error, but then can calibrate their own exposure dosages.