I know this post is late, but I hope it is worth the wait (if anyone was waiting 
). My energy levels were just not sufficient for me to write anything coherent at the end of the fourth day of conference, but I am feeling better now that the conference has ended. So let’s begin with events of Wednesday.
Wednesday’s program convinced me that parallel universes do exist. I was shuttling between ground floor and first floor (aka banquet floor) trying to pop-in and pop-out of talks at right times. Keeping with theme of last post, I will point out one talk each from above three areas. As a bonus, I also point out how my talk went.
Computational imaging:
The first session of the day that I went to was the joint interdisciplinary session of AO/COSI/SRS. The talk that I remember clearly is Marc Levoy’s on light-field photography and microscopy (JWA3). Marc has visited Singapore earlier and given a similar talk. So he had advised me against attending his talk and spend time gathering something new. But it was good to see how he is planning to apply these methods to decoding of neuronal wiring in zebra fish (or another?) embryos. The idea is to stimulate neurons located at arbitrary 3D locations using light-field illumination architecture. This ’stimulate firing’ of neuron should be followed by fast 3D imaging of activation of neighboring neurons. Yes, the neurons have to be labelled correctly – you need to label neuronal ion channels (rhodopsin) in such a way that when the label is excited the channel opens. Plus, all neurons have to be labelled with calcium indicator dye which indicates when certain neuron becomes active. The enabling technology in labeling is genetic modification of the zebra fish to express these proteins in live animals.
Phase retrieval:
The next noteworthy talk I attended was Greg Gbur’s (SWA1) on intensity diffraction tomography. The idea is rather neat – you measure the phase of the specimen by tomography but without explicitly measuring phase (e.g. by interferometry or otherwise). The idea relies on the fact that phase information of the specimen does affect intensity of diffracted light and hence one ought to be able to retrieve the phase distribution from intensity measurements alone. I should note that similar goal is achieved in a different way by transport of intensity formulation.
In the same session was my talk about registration of gradient information (SWA4). We know that traditional methods of image registration are meant to register measurements of the same function. Therefore, when we want to register gradients of some function along X and Y directions, these methods don’t work. I figured that registration of gradients requires that you reformulate the registration problem and devise a new procedure for computing the registration information. This registration has allowed me to accurately reconstruct phase information from phase-gradients that I measure with differential interference contrast (DIC) and differential phase contrast (DPC). I did better at this talk than the one on Monday and had some useful discussion after it. I was helped by the fact that the speaker after me had withdrawn, giving me more time to delve onto details.
Phase-space analysis:
It was pointed out by Prof. William Rhodes that Wigner distribution computed from discrete samples of a signal may contain erroneous structures (FWW1). This happens because when a temporal signal is sampled, it becomes periodic in frequency with the period equal to the sampling frequency. These periodic components of frequency produce cross-terms. These cross-terms cancel out if one projects the signal along frequency (to obtain signal intensity) but do not if one projects the signal along space (to obtain spectrum density). These errors are exacerbated when one operates on this distribution (e.g. propogate by shearing) and then again tries to compute intensity or spectral density by projection.
Categories: FiO09 · imaging · microscopy · optics
Tagged: gradient registration, light-field, phase-retrieval, phase-space, sampling
Recently, I have got interested in spatial light modulators (SLMs). SLMs are to optics what programmable logic is to computing. They allow modulation of certain properties of light (e.g., amplitude, phase, polarization). I thought of noting down my finds about available technologies and update them as I stumble upon new information. A good question to ask is – ‘what do we precisely mean by modulation’? Modulation here means changing two dimensional distribution of amplitude, phase or polarization of light at given plane. I am particularly interested in amplitude modulators for automating our new quantitative imaging method based on oblique-illuminaiton. Here are links to our conference abstracts and a journal paper:
I classify the spatial light modulators depending on the type of modulation they are designed to provide. Most of the time, one would like to modulate either amplitude, phase, or polarization without affecting the other properties. In basic optics research, SLMs are widely used for phase modulation in areas of holography, optical communication, optical trapping, beam shaping, adaptive optics etc. In commercial applications, amplitude modulation dominates where SLMs are used for writing dynamic patterns that are projected. Polarization modulation has been used in design of programmable spectral filters called Lyot filter and quantitative birefringence imaging devices called Polscope. As noted next, all SLM technologies except for digital micro-mirror devices (DMD) and deformable mirrors employ birefringence modulation under electronic control for achieving amplitude, phase or polarization modulation. Most of the birefringent reflective devices seem to use LCoS (liquid crystal on silicon) technology. Reflection based geometry allows putting control logic behind the liquid crystals leading to high density and possibility of calibration. In transmission SLMs, the logic is implemented around the pixels which limits amount of intelligence (e.g. calibration circuit) that can be built in and causes spurious diffraction.
Amplitude modulators:
If you want to perform binary modulation (black and white), digital micro-mirror devices (DMD) and ferroelectric liquid crystals (FLC) are the technologies of the day. For grayscale amplitude modulation, twisted nematic liquid crystal (TNLC) devices seem to be the most suitable.
- DMDs are micro-mechanical devices in which a tiny mirror is mounted on a semiconductor chip whose orientation is controlled by currents produced on the chip under programmable control. Texas Instruments invented DMDs for projection applications with goal of substituting the roll of film by this single device. DMDs are fast and one can update the patterns at the rate of MHz. Advantages of DMDs include nearly-polarization-insensitive modulation, fast switching time, high damage threshold, and low cost. But, they are bit awkward to use because of their reflection based geometry which requires oblique illumination to separate unmodulated and modulated light. There are some prism-based optical modules noted on TI website which provide easy integration of DMD in light path.
- FLC devices use bistable liquid crystal whose birefringence can be switched at MHz rate. By placing FLC between two crossed or parallel polarizers, one can perform binary modulation. They are little pricier than DMDs and available from Displaytech. Depending on your application, FLCs polarization sensitive operation may be an advantage or disadvantage. They operate at lower power than DMDs and allow smaller pixel sizes. They can be used in reflection (which is frequent) or transmission.
- Both types of black and white technologies can mimic grayscale modulation due to their fast switching speed. Grayscale modulation can be achieved by temporal modulation at rates much faster than integration time of the detector. This works fairly well with commecial projection applications as visual integration time is around 33ms (1/30 s).
- True grayscale modulation can be obtained with twisted-nematic liquid crystal (TNLC) devices, which again can be used in transmission or in reflection between polarizers. Holoeye and Meadowlark both supply twisted-nematic LCDs. Holoeye is very active in design and support of TNLC SLMs. Their reflection-type device LC-R 1080 is based on liquid crystal on silicon (LCoS) technology and designed for amplitude modulation. Most of the commercial displays are based on TNLCs, so you might source one from an unused display projector. Holoeye provides an OEM kit (HEO 0017) based on a commercial display from Sony and costs nearly half in comparison to its reflection devices.
Phase modulators
- (Thanks to Laura, friend at MIT, for reminding me about deformable mirrors) The most extensively used devices for phase-modulation are deformable mirros (DM). There are several technologies for realizing DMs. Conceptually, DM can be thought of as a flat stretched reflective membrane mounted on actuators. The actuators deform the membrane as per required phase modulation. They are widely used in astronomy, retinal imaging, microscopy research, and pulse shaping. Several companies provide DM and phase-modulation kits suitable for above application, a noteworthy among them being Boston Micromachines.
- With TNLCs, one can control the phase distribution of light but perhaps accompanied by spurious amplitude modulation. However, Holoeye combines twisted nematic LCs with intelligent voltage control to negate spurious amplitude modulation with its pluto and HEO 1080P devices.
- A competing technology is Parallel aligned liquid crystal (PALC). PALCs twist along the propagation axis rather than around it. They do not rotate polarization of input light but merely change refractive index seen by it. Hence, they allow pure phase modulation. Such devices are available from Hamamatsu.
Polarization modulators
- These modulators are usually used for variable retardation, i.e. to control ellipticity of light. TNLCs are again widely used for this purpose. In fact, the basic mechanism of TNLC modulation is to alter the 2D distribution of polarization of light, which is then converted to amplitude or phase modulation with help of polarizer on the output side. Arcoptix and Meadowlark supply devices meant for programmable retardation, generation of radially or azimuthally polarized light, etc.
Categories: imaging · microscopy · optics
Tagged: SLM, spatial light modulator
