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Texas Instruments DLP® NIRscan™ Nano Optical Design Considerations Application notes
Application Report
DLPA062 – January 2016
Texas Instruments DLP® NIRscan™ Nano Evaluation
Module (EVM) Optical Design Considerations
Gavin Perrella
ABSTRACT
TI DLP® technology offers unique benefits and functionality to spectrometer design. For a comparison of
DLP spectroscopy to existing technologies see Texas Instruments DLP Technology for Spectroscopy
(DLPA048). The optical design of a DLP spectrometer involves interactions between many interdependent
parameters. Modification of individual components often requires the simultaneous alteration of numerous
other elements in order to maintain desired performance levels. This application report identifies the key
optical parameters, and discusses their interdependent impact on system size and performance. The
design choices for the DLP NIRscan™ Nano EVM are explained, and from these, generally applicable
design guidelines are derived.
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Contents
System Layout ...............................................................................................................
Illumination Module ..........................................................................................................
Fiber Coupled Illumination ..................................................................................................
DMD F-Number ..............................................................................................................
DMD Illumination Angle .....................................................................................................
Slit Parameters ...............................................................................................................
Spectral Resolution ..........................................................................................................
Diffraction Grating ...........................................................................................................
Condenser and Detector ....................................................................................................
Optical Lens Elements ......................................................................................................
Stray Light ....................................................................................................................
Opto-Mechanics ..............................................................................................................
Modifying the NIRscan Nano EVM ........................................................................................
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List of Figures
..............................................................................
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NIRscan Nano EVM With Cover Removed
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Four Lamp Diffuse Reflectance Illumination Module .................................................................... 3
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Slit Fill Factor with Standard Fiber
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Slit Fill Factor with Round-to-Linear Fiber ................................................................................ 4
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NIRscan Nano EVM DMD Illumination Angles........................................................................... 5
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List of Tables
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NIRscan Nano EVM Design Specifications
..............................................................................
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NIRscan is a trademark of Texas Instruments.
DLP is a registered trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
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Design Considerations
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1
System Layout
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System Layout
The NIRscan Nano EVM system layout is shown in Figure 1. During typical use, the sample under test is
pressed against the sapphire window and illuminated by two broadband tungsten filament lamps. Light
diffusely reflected by the sample is gathered by the collection lens and focused through the slit. The
collected light is collimated by the first set of lenses, passes through a longpass filter, and then strikes the
diffraction grating which disperses the light into its constituent wavelengths. The longpass filter blocks light
with wavelengths shorter than 900 nm from striking the grating. Otherwise, the second diffraction order of
this light would overlap with the first order of the 900-nm to 1700-nm wavelength range, reducing the
signal-to-noise ratio (SNR) of the system. The focusing lenses then form an image of the slit at the digital
micromirror device (DMD), with the different wavelengths spread across the length of the DMD. Sets of
DMD mirror columns are turned on in sequence to reflect one wavelength of light at a time to the
condenser lenses, which concentrates this energy onto the single point InGaAs detector. Table 1 displays
the primary design specifications of the NIRscan Nano EVM.
Detector
Longpass
Filter
Grating
Focusing
Lenses
Condenser
Lenses
Collimating
Lenses
Slit
DMD
Collection
Lens
Lamp
Lamp
Sapphire
Window
Figure 1. NIRscan Nano EVM With Cover Removed
Table 1. NIRscan Nano EVM Design Specifications
PARAMETER
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DESIGN VALUE
Slit ƒ-number
2.5
Slit dimensions
1.8 × 0.025 mm
DMD
DLP2010NIR
DMD ƒ-number
3.8
Spectral range
900 to 1700 nm
Spectral resolution
10 nm
Detector diameter
1 mm
Detector type
Uncooled InGaAs
Optical engine dimensions
33 × 29 ×10 mm
Texas Instruments DLP® NIRscan™ Nano Evaluation Module (EVM) Optical
Design Considerations
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Illumination Module
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2
Illumination Module
The NIRscan Nano EVM is equipped with a removable diffuse reflectance illumination module. This unit
operates by illuminating the sample under test at an angle so that specular reflections are not collected,
while diffuse reflections are gathered and focused through the slit. Two lens-end, tungsten filament lamps
serve as the light source. They are designated as lens-end lamps because the front end of the glass bulb
is formed into a lens to direct more light from the filament to the sample test region. The collection lens
gathers collimated light from a 2.5-mm diameter region at the sample plane. The size of the collection
region is matched to the nominal illumination spot size created by the lens-end lamps.
The NIRscan Nano EVM illumination module is designed for the sample to be placed directly against the
sapphire window, where the two angled light source paths and the vision cone of the collection lens all
intersect. If the sample is shifted farther away from the window, the system may not collect enough light to
perform an accurate measurement. If sample contact is not desirable, the illumination module could be
redesigned to accommodate non-contact measurements. The position and angle of the lamps could be
changed to illuminate a point in space. In such a setup, it is more difficult to prevent stray light, such as
room lights, from entering the spectrometer and impacting the accuracy of its measurements.
There are also methods of altering the illumination module to increase the signal strength of the
spectrometer input. One approach is to make the module an imaging relay, instead of collecting a
collimated bundle of light from the sample. This would increase the collection efficiency at the sample,
while simultaneously reducing the size of the sampling region, making non-homogeneous samples difficult
to measure accurately.
An alternative solution is to increase the illumination source amplitude by increasing the quantity, power,
or size of the lamps. An example of this is the four lamp illumination module shown in Figure 2.
Additionally, elliptical reflector housings or focusing lenses could be added to collect more light from each
lamp.
Figure 2. Four Lamp Diffuse Reflectance Illumination Module
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Fiber Coupled Illumination
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Fiber Coupled Illumination
Optical fibers are often used to couple different illumination modules with the main spectrometer system.
DLP spectrometers will typically have taller entrance slits than array detector systems because the DMD is
taller than the array detector. This allows for increased signal throughput, but can make fiber coupling
more complex. Standard optical fibers can be utilized, but will only illuminate a small section of the
entrance slit, as shown in Figure 3. In order to take advantage of the DLP spectrometer’s large etendue, a
round-to-linear fiber bundle should be used. These optical fibers have multiple cores arranged in a circular
pattern at one end, and a linear configuration at the opposite end. The round end can be utilized at the
sample as in typical spectroscopy illumination modules, while the linear end can be more effectively
coupled to the slit, as shown in Figure 4.
Fiber
Slit
Figure 3. Slit Fill Factor with Standard Fiber
Fiber Cores
Slit
Figure 4. Slit Fill Factor with Round-to-Linear Fiber
Other important considerations for fiber coupling are numerical aperture matching and fiber to slit
placement. To maximize fiber coupling efficiency, the numerical aperture (NA) of the fiber output should
be no larger than the acceptance NA of the spectrometer system. The NIRscan Nano EVM entrance slit
operates at 0.2 NA, which matches many standard optical fibers. The most basic fiber coupling setups
place the fiber end directly at the slit entrance with as minimal a gap as possible. Any separation between
the fiber and slit rapidly decreases the coupling efficiency. If the fiber cannot be placed at the slit due to
mechanical restrictions, it is possible to design an optical relay which images the output of the fiber to the
entrance slit plane. Such a module could replace the standard diffuse reflectance module on the NIRscan
Nano EVM.
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DMD F-Number
While the DLP2010NIR DMD accepts up to an ƒ/1.7 cone angle, the NIRscan Nano EVM operates at
ƒ/3.8 at the DMD. This slower ƒ-number offers a compromise between light throughput and engine
complexity. Faster ƒ-numbers allow the spectrometer to collect more light from the sample and increase
the diffraction efficiency of the DMD, ultimately resulting in a higher SNR. The tradeoff for these advances
comes in terms of optical engine size and complexity. The lenses must increase in size in order to
accommodate the larger light cone. Operating at a faster ƒ-number may cause a degradation of the optical
image quality at the DMD plane unless additional lenses or aspheric lens surfaces are added. Any
resulting decrease in the slit image quality at the DMD will reduce the spectral resolution of the device.
Faster ƒ-numbers may also necessitate total internal reflectance (TIR) or reverse total internal reflectance
(RTIR) prism arrangements at the DMD in order to separate the incoming and outgoing light bundles.
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DMD Illumination Angle
The DLP2010NIR device is typically illuminated at a 34° angle of incidence, resulting in the reflected light
leaving normal to the DMD. However, the dispersive properties of the diffraction grating and lens elements
cause the dispersed slit image to form along a tilted plane not perpendicular to the optical axis. The tilt
angle of this plane will vary widely between different designs and may not match the desired 34° angle of
incidence. For instance, the NIRscan Nano EVM design creates the dispersed slit image at a 10° angle.
The image plane angle can be adjusted by the addition of a wedge prism element, or by tilting and
decentering the focusing lenses between the grating and DMD.
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Design Considerations
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Slit Parameters
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Instead of altering the 10° image plane angle, the NIRscan Nano EVM accommodates it by reversing the
DMD illumination path, as shown in Figure 5. The moderately slow ƒ-number at the DMD, in conjunction
with the shallow angle of incidence, allow the incident and reflected light to remain within the angular
usage range over which the DLP2010NIR DMD was designed. This means the DMD window aperture will
not obstruct the light path. The 17° pixel tilt of this device also aids in separating the incident and reflected
light cones. This allows the lenses to be placed closer to the DMD without any mechanical interference,
keeping the system more compact.
Figure 5. NIRscan Nano EVM DMD Illumination Angles
When reversing the DMD illumination path it is also important to consider the off-state light. The
DLP2010NIR DMD utilizes perpendicular tilt directions for on-state and off-state pixels. This architecture
directs off-state light away from the main optical path, even when using a reverse illumination scheme,
where it can be blocked by proper chassis and baffle design.
This configuration illuminates the DMD mirrors from an angle opposite their on-state tilt direction, which
increases the gap between adjacent mirrors from the perspective of the incoming illumination. More light
will therefore be absorbed underneath the mirrors, instead of reflected on towards the detector. It is
important to take this into consideration when estimating total system efficiency to ensure the final design
meets SNR and measurement performance goals.
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Slit Parameters
The input slit parameters impact the SNR and spectral resolution capabilities of the system. The NIRscan
Nano EVM possesses an ƒ/2.5 acceptance cone at the slit because this matches many standard fiber
optic bundles, allowing for easier coupling with fiber illumination modules. The ratio between slit and DMD
ƒ-number sets the magnification of the system, which is 1.52 for the NIRscan Nano EVM. Knowing the
system magnification, one can then determine an appropriate slit width and length. The slit width should
be chosen to create an image width at the DMD corresponding to a desired spectral resolution, while the
length should be large enough to illuminate the full extent of the DMD panel, maximizing the system’s light
throughput. Narrowing the slit width increases the spectral resolution of the system while simultaneously
reducing the SNR since less light can be collected from the sample under test. The optical system must
be capable of sharply imaging the smaller slit to the DMD in order to realize this gain in spectral
resolution.
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Spectral Resolution
The spectral resolution of the NIRscan Nano EVM is defined as the spectral full width at half maximum
(FWHM) of a measurement taken using an input source with zero spectral width. This can be modeled in
optical design software by filling the input slit with a monochromatic light source and measuring the
resultant FHWM of the image at the DMD plane. This method of analysis shows that the NIRscan Nano
EVM achieves a 10-nm spectral resolution with its 25-μm wide slit. Thinner slits could be used to achieve
finer resolution at the cost of SNR. It is also important to consider the resolution of the DMD to ensure it
can adequately sample the spectral image of the slit. The FHWM of a monochromatic slit image in the
NIRscan Nano EVM subtends nine DMD pixels, ensuring this is not an issue.
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Diffraction Grating
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Diffraction Grating
Diffraction grating selection influences the system size and layout. A tighter groove spacing (increased
grooves per mm) will increase the divergence angle variation between different wavelengths. This
separates the broad spectrum light over a shorter distance, allowing the DMD to be positioned closer to
the grating and still capture the entire spectral range. Increasing the grating frequency also degrades the
optical image quality at the DMD, increasing the difficulty of the optical engine design. Varying the groove
spacing allows one to balance the system size and optical performance.
Another important consideration is how the dispersed spectrum fills the DMD array. Typically, the DMD is
underfilled in the dispersion axis, and overfilled in the orthogonal axis, to allow for image location shifts
caused by optical and mechanical tolerances. For a more in-depth explanation of these effects, see Texas
Instruments DLP Spectrometer Design Considerations (DLPA049).
9
Condenser and Detector
The condenser lenses serve to collect the light from any DMD mirrors in the on-state and concentrate it
onto the single point detector. The smaller the region that all wavelengths are condensed into, the smaller
the detector that can be used. Shrinking the detector size has two main benefits - decreasing the cost,
and increasing the SNR. In order to accomplish this, the condenser should operate at a fast ƒ-number to
create a greatly de-magnified image of the DMD at the detector plane. This portion of the system is
optimized to improve collection efficiency and energy distribution uniformity at the detector over imaging
performance. The NIRscan Nano EVM employs purposeful defocus at this image plane by shifting the
detector position towards the condenser lenses. The detector is placed where the light bundle is most
narrow to balance the amount of light collected across all wavelengths.
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Optical Lens Elements
Lens material selection plays an important role in the optical design. All the NIRscan Nano EVM lenses
are made of Ohara S-LAM66 and have only spherical and flat surfaces. This glass is relatively common
and inexpensive, while still having a high index of refraction and physical properties conducive to standard
manufacturing techniques. The total lens count could be reduced through the use of aspheric optics made
of a lower index moldable glass. This approach could help to reduce the size of the system and the
complexity of the mechanical chassis. However, molding custom aspheric glass lenses is only viable in
terms of cost at large manufacturing quantities. Plastic lens materials are not suitable due to absorption
bands within the 900-nm to 1700-nm range.
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Stray Light
Stray light refers to errant signals caused by incorrectly classifying the wavelength of a signal. This causes
erroneous spectrum measurement output, and limits the linearity of absorbance units (AU) that can be
accurately measured. For an in-depth treatise on potential stray light sources, refer to Texas Instruments
DLP Spectrometer Design Considerations (DLPA049).
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Opto-Mechanics
It is important to consider mechanical alignment tolerances when performing the optical design. Slit and
detector position may be actively aligned to compensate for tolerance errors and increase system
performance. For further information on opto-mechanical design considerations, refer to Texas
Instruments DLP Spectrometer Design Considerations (DLPA049).
6
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Design Considerations
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Modifying the NIRscan Nano EVM
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13
Modifying the NIRscan Nano EVM
Due to the interdependent nature of the system’s optical design parameters, only a few are well-suited to
customization. Replacing the illumination module or entrance slit offers an effective path for adapting the
spectrometer to different applications. The number, type, and arrangement of lamps can be altered to
increase the light output or accommodate different sample positions, as shown in Section 2. A
transmissive or fiber coupled illumination module could be used instead of the standard diffuse reflectance
setup. More details on fiber coupled illumination can be found in Section 3. Adjusting the width of the
entrance slit allows resolution and SNR to be traded off against each other, as shown in Section 7. The
single point photodetector could also be exchanged with another as long as it still fits within the
mechanical housing constraints.
Modifications to the illumination module, entrance slit, and detector of the NIRscan Nano EVM can be
performed without any changes to the core system. More extensive alterations such as shifting the
spectral measurement band or adjusting the spectral resolution further than exchanging the entrance slit is
capable of will require redesigning the entire optical path and mechanical chassis. The NIRscan Nano
EVM design files serve as a reference and starting point for such endeavors.
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