Typical Values
Optical Properties
| Property | Test Condition | Value | Unit | Standard |
|---|---|---|---|---|
| Typical substrate thickness | 60 | microns | acc. to ISO 4593, 23°C | |
| Typical photopolymer thickness | 30 | microns | IR Interferometer | |
| Typical cover layer thickness | 50 | microns | acc. to ISO 4593, 23°C | |
| Typical recording dosage needed to achieve above ∆n1 values | See Annex graph 2, for details of the recording dosage | mJ/cm² | ||
| Light transmittance | (Unrecorded film, w/o cover foil) | See Annex graph 1, for details of the transmittance spectrum | % | ASTM E 01348 |
| Maximum refractive index modulation ∆n1 per recording wavelengt λ | acc. to ISO 17901-2 | |||
| λ = 633 nm | > 0,04 | |||
| λ = 532 nm | > 0,04 |
Storage
The unrecorded photopolymer film should be stored in the original and sealed Covestro packaging that is used for delivery. The unrecorded photopolymer film should be protected from light, humidity, heat , foreign materials and compressive load.Recommended storage temperature: +15 °C to +25 °C.Recommended relative humidity 45 % to 55 %
Storage time
Storage time 6 month.
Processing
Reflection Holograms in 2-Beam GeometryIn this recording geometry the photopolymer film is laminated onto a glass plate (75 mm x 50 mm x 1 mm). The holographic recording is done with two coherent and collimated laser beams (plane waves), which penetrate the prepared sample from its two different surfaces. Both laser beams have S-polarization to maximize the interference contrast (fringe visibility) of the holographic recording.The holographic recording itself is done with dosages which correspond to the product of the total incident power density (PR + PS) multiplied by the individual exposure times, t. More detailed conditions are depicted in the two following figures.The ratio (PS / PR) = 1.29 compensates for the different size of the projected beam cross sections onto the sample surfaces and the different losses due to Fresnel reflections at the air sample interfaces in such a way that inside the photopolymer layer the beam ratio is equal to 1. This again facilitates maximum fringe visibility during the holographic recording.After the holographic recording, the material is bleached using a dosage of approx. 5 J/cm² in the UV spectral range and simultaneously approx. 5 J/cm² in the visible spectral range.
To determine the performance of the hologram, the diffraction efficiency η of the recorded hologram is measured with respect to the rotation angle α (the holographic film / glass sandwich is mounted on a rotation stage). The resulting Bragg curve η(α) is analyzed according to the Kogelnik theory* , to deduce the performance parameters ηmax, Δn1 and the effective thickness shrinkage. The effective thickness shrinkage is obtained from the angular shift ε, which measures the deviation from the recording angle α to the angle at which ηmax is achieved. This thickness shrinkage is incorporated in the fit of Δn1 according to Kogelnik theory.The mean refractive index used for the calculation is 1.51.By varying the recording times the dosage response curves for Δn1 at the different recording wavelengths, λ = 633 nm, 532 nm and 457 nm, can be obtained. Source: *Kogelnik, H., "Coupled wave theory for thick hologram gratings", The Bell System Technical Journal 48(9), 2909-2947 (1969) Graph 1: Transmittance Graph 2: Index Modulation
