UltraFlat Dichroics

For distortion-free reflection of lasers

Chroma manufactures UltraFlat dichroics for applications demanding superior levels of surface
flatness for laser applications such as TIRF and other super-resolution techniques. We offer UltraFlat
dichroics either for use as individual parts or included in assembled filter sets.
However, the secret with flat dichroics is that they need to stayflat in your imaging system. Filter cubes that use clips, springs or
screws to hold dichroics will torque them out of flatness. Chroma's
TIRF & Super-resolution filter cubes keep your dichroics just as flatin your microscope as they were when we made them.
In this context, surface flatness typically relates to curvature,and describes how curved the dichroic surface is. Surface
curvature causes convergence or divergence of reflected
light waves, depending on whether the surface is concave orconvex. This results in reflected wavefront distortion (RWD)
of whatever is being reflected: lasers, both in basic imaging
applications and in more advanced methods such as TIRF andSTED; structured illumination patterns; and reflected images inimage-splitting systems.
Sputtered thin-films exert stress on glass and fused silica
substrates and warp them into varying degrees of curvature.
Chroma has learned how to control this to a large extent
by developing a proprietary manufacturing method which
minimizes surface curvature. Another factor which reduces
surface curvature is the use of thicker substrates which provide
greater stiffness and therefore more resistance to the stressexerted by these coatings.

Combining these two elements allows Chroma to specify levels of
dichroic surface flatness according to thickness. We offer Chroma’s
UltraFlat dichroics with the following specifications for final, postcoating surface flatness:
Thickness

Surface Flatness

Application

1mm thick:=/< 2 waves/inch
Peak-Valley (P-V)

Standard Laser FilterSets

2mm thick:=/< 0.5 waves/inch P-V

TIRF Filter Sets, PALM
and STORM

3mm thick:=/< 0.25 waves/inch P-V

STED and Structured
Illumination=/>5mm thick:Contact us

Custom Applications

The surface flatness of each lot of our UltraFlat dichroics ismeasured using laser interferometry. Possibly even more
important regarding flatness is how the dichroic is held or housed.
Even the flattest optics are warped by varying degrees when heldin place by mechanical means. See “Holding Dichroics” below.
Note: Our catalog dichroics for basic epifluorescence widefield applications
are not controlled for flatness because widefield illumination does not
require it. Please specify when you require our UltraFlat dichroics.

CHROMA TECHNOLOGY CORP®
Contact: sales@chroma.com or +1-800-824-7662 • +1-802-428-2500 · www.chroma.com

Holding Dichroics
The manner in which dichroics are held or housed in filter cubesdramatically affects their actual flatness in real world applications.
Any mechanical means of holding a dichroic will introduce somedegree of pinching or twisting which invariably results in warpingof the surface of the dichroic, reducing it's flatness. In addition to
any spherical curvature, this will contribute irregular deviations
from surface flatness.
Major microscope manufacturers generally specify 1mm thickdichroics for their standard filter cubes, and these are often heldin place mechanically, by springs or clips. Often, this is sufficientfor holding 1mm-thick dichroics flat enough for routine laser
applications such as confocal or epi-fluorescence using laser
illumination, photo-activation and laser ablation. Our 1mm thick
UltraFlat laser dichroics measure <2 waves/inch Peak-Valley
surface flatness, providing the performance needed for routine
laser use.
For more demanding laser applications such as TIRF andother super-resolution techniques, as well as applications
where images are reflected to a camera or to other optics, ourthicker UltraFlat dichroics can provide much better results. Inorder to optimally hold these dichroics, Chroma has designed andmanufactured metal microscope cubes which fit most current
microscope models and can accommodate dichroics up to 3mmthick. These cubes affix the dichroics without the use of springs,
clips or screws and are aligned at Chroma using set screws to a
precise 45 degree angle of incidence.
For workers with their own holders or mounts, we recommendthat you hold by placing minimal pressure on the outside edges,
rather than by pinching on the top/bottom surfaces to minimize
warping. Call or email us to discuss the range of sizes andthicknesses we can provide.

How We Specify Surface Flatness
The surface flatness parameter we measure is referred to as“Peak-to-Valley” (P-V) deformation, and is expressed in “waves/inch” (or lambda/inch) as determined by laser interferometry.
This measures the maximum deformation across the clear
aperture of a dichroic, and includes the curvature (Power) plus
any surface irregularities.
Industrial standards for surface flatness measurements offlat optics, such as dichroics, conform to ISO standards, andare expressed in terms of interferometric “fringe spacings”
or fringes. These are interference patterns which appear as aresult of differences in index of refraction between that of thedichroic substrate material and air as a laser is reflected off of themeasured surface.
The number of fringes is used to calculate the deviation of themeasured surface from that of a reference optical “flat”. We
measure this using a wavelength of 633nm, which is the laser
most often used in an interferometer.
Occasionally, a filter manufacturer may express surface flatness
in terms of radius of curvature (ROC), which in the context of flat
optics is a more obscure and confusing metric. ROC is used mainly
by lens manufacturers who deal with relatively large values
for curvature. As an example of how our flatness specification
relates to ROC, consider that a 0.5 wave/inch surface flatness isequivalent to a radius of curvature of 254 meters (or about 830
feet). ROC defines the actual radius of a sphere which surface
curvature is identical to the curvature of a surface of an optic.
Others prefer the parameter of “RMS” (root mean square) which
provides a measurement of the uniformity of the surface. Because
any distortion to surface flatness as a result of the thin film
coatings we use will be spherical distortion, this means that theRMS value will typically be approx. =/< ¼ of the of the P-V value.
“RMS” will result in a smaller value than P-V to describe the surfaceof the same dichroic or mirror.
For the same surface curvature, the various measured values forthese parameters will typically vary in the following way:
P-V > Power >> RMS.
Sometimes, P-V flatness is defined over a smaller area, such as a
10mm or 15mm clear aperture. The values listed above use the larger
scale of 1 inch which results in a larger value for the same curvature.
The relationship between measurement length and flatness
is non-linear. Assuming the deformation is primarily sphericalcurvature due to coating stress, this can be described by asimple quadratic formula. To calculate the equivalent flatness
for a clear aperture of ½ the measured value, the flatness
expressed as number of waves will be ¼ of the measured value.
As the denominator in the expression (inches) varies by “x”, thenumerator (waves) varies by x2. An optic which measures 2 waves/inch P-V will measure 0.5 waves/0.5 inch P-V.
If you prefer the surface flatness expressed as Surface Power orRMS, we will provide this upon request.
Finally, remember that the method of holding or housing thedichroic will greatly influence its actual flatness when used in animaging system.

CHROMA TECHNOLOGY CORP®
Contact: sales@chroma.com or +1-800-824-7662 • +1-802-428-2500 · www.chroma.com