Spatial light modulator (SLM) is a general term describing devices that are used to modulate amplitude, phase, or polarization of light waves in space and time. HOLOEYE´s Spatial Light Modulator systems are based on translucent (LCD) or reflective (LCOS) liquid crystal microdisplays.
The use of LC materials in SLMs is based on their optical and electrical anisotropy. A certain gray level represents a defined average voltage across the LC cell. This voltage leads to a variable tilt of the LC molecules due to their electrical anisotropy. As LC molecules also show optical anisotropy this tilt changes the refractive index of the LC molecules (for suitable incident polarization, dependent on device version) which causes a modified optical path length within the LC cell. The addressed gray level is now converted into a phase level.
HOLOEYEs SLMs are based on vertical aligned nematic (VAN),parallel aligned nematic (PAN) or twisted nematic (TN) microdisplay cells. In a twisted cell, the orientation of the molecules differs by typically 45°/90° between the top and the bottom of the LC cell and is arranged in a helix-like structure in between. In VAN / PAN cells the alignment layers are parallel to each other, so the LC molecules have the same orientation.
To use an SLM in amplitude modulation mode you need linear incident polarisation. The transmitted or reflected light has to be guided through a 2nd polariser (analyser) that is crossed to the incident polarisation. For phase modulation a setup without an analyser is used. With devices based on twisted nematic LC or LCOS displays the twist always causes a polarisation effect (amplitude modulation) and no phase only modulation is possible (phase mostly modulation).
With VAN / PAN displays it is possible to modulate the phase without influence on the polarisation / amplitude (phase only modulation) by using incident polarization along the LC director axis.
HOLOEYE Spatial Light Modulators:
- Resolution: 1920 × 1080
- Pixel Pitch: 4.5 µm
- Fill Factor: 91 %
- Wavelengths Range: 420 – 650 nm,
The LUNA Spatial Light Modulator is our most compact SLM platform for integration into small sized or even portable solutions.
- Resolution: max. 4160×2464 (4000×2160)
- Pixel Pitch: 3.74 µm
- Fill Factor: 90 %
- Wavelengths Range: 420 – 1100 nm (Different Versions),
The GAEA is currently the highest resolution SLM on the market with extremely small pixel pitch.
- Resolution: 1920 x 1080
- Pixel Pitch: 8.0 µm
- Fill Factor: 93 %
- Wavelengths Range: 350 – 1700 nm (Different Versions)
The PLUTO-2.1 SLM is our all-rounder within our product range. It is the best qualified and diversified SLM platform with many versions optimized for specific requirements. Also PLUTO is already implemented in many industrial applications.
- Resolution: 1920 x 1080
- Pixel Pitch: 6.4 µm
- Fill Factor: 93 %
- Wavelengths Range: 420 – 1100 nm (Different Versions)
The LETO-3 is our fast SLM platform with high band width. The SLM is capable of color sequential phase operation.
- Resolution: 1024 x 768
- Pixel Pitch: 36 µm
- Fill Factor: 58 %
- Wavelengths Range: 420 – 800 nm
Definitions of Specs
- Resolution: Number of pixels (width x height)
- Pixel Pitch: Size of a pixel including the interpixel gap
- Fill Factor: Surface area of the display which can actively used. There are gaps between the pixels at which the incident light is scattered.
- Active Area: Size of the actual adressable/usable display area.
- Addressing: Number of gray levels / phase levels that can be addressed. This can vary with addressing sequences.
- Signal Formats: Input signal format. Typically HDMI or DVI.
- Input Frame Rate: Addressing speed of the input signal (typically DVI / HDMI video frame rates of 60 Hz for monochrome applications).
- Response Time: The response time is defined as the switching time from 10% to 90% and from 90% to 10 % (rise and fall time). The actual response time of the liquid crystal is determined by the properties of the used liquid crystal material, the thickness of the LC layer, the used drive sequence / calibration (the actual voltages applied to a pixel) and temperature.
For phase SLMs the response time typically is below the Input frame rate.
- Reflectivity: Amount of light which is directly reflected (0-order of a non-addressed display). The reflectivity is not 100% as some of the light is diffracted into higher orders due to the grating like structure of the pixel matrix. Some part of the light is also scattered and absorbed at the interpixel gaps. In addition the reflectivity of the aluminum mirror is limited (dependent on wavelength).
Addressing the SLM:
The optical function or information to be displayed on HOLOEYE Spatial Light Modulators can be taken directly from an optic design software or an image source and can be transferred by a computer interface. Using DVI/HDMI ports of standard PC graphics cards, the Spatial Light Modulator can be used just like an external plug & play monitor (e.g. in extended desktop mode for a second monitor) and no special software or drivers are necessary to operate the SLM. No matter which software is used to calculate optical functions, if you are able to address a bitmap image on a second monitor output of the graphics card you are able to operate the SLM. Also standard image viewer software can be used.
For an easy start and even more convienent operation HOLOEYE provides a software package with each SLM. It contains a Pattern Generator Software (for calculation of different optical functions), a Slideshow Player software (for easy addressing of precalculated functions or images on the SLM) and an SLM Display SDK for different development environments. Also a Configuration Manager software is included for convenient configuration of the Spatial Light Modulator.