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Optical filters
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mounts
About Optical Filters
Optical filters selectively pass (transmit) light of specific wavelength
or range of wavelengths while blocking the remainder. For example longpass
filters pass long wavelengths only , shortpass pass short wavelengths
only, bandpass pass a band of wavelengths. The passband may be narrow
or wide depending on the filter type and the transition or cutoff between
maximal and minimal transmission can be sharp or gradual.
Optical filters can be used to increase the contrast of features of interest
in a scene in order to make it simpler for imaging software to detect
and measure. For example you may choose to use a monochrome camera because
it runs faster or gives sharper images due to lower chromatic aberration.
However you need to pickup a blue feature on a red background and the
monochrome image provides very little contrast between these two colours
because their intensity levels may be very similar . A shortpass filter
with its crossover between red and blue will pass the blue and block the
red so the blue feature will appear bright and high contrast aganist the
red feature which is blocked and appears dark.
The simplest types of filters are made of absorptive materials (absorptive
filters) while the more complex and higher-performing dichroic (interference)
filters use multiple layers of insulating materials deposited on a substrate
which causes constructive/destructive interference of the light in order
to achieve the required performance. Dichroic filters tend to have sharper
transitions and lend themselves more to narrow band filtering.
Filter Types
Bandpass filters: BP filters transmit
all wavelengths within a range, and block others.
Longpass filters: LP filters transmit light with wavelengths
longer than the cutoff (transition) wavelength
Shortpass filters: SP filters transmit light with wavelengths
shorter than a cutoff wavelength
Neutral Density filters: ND filters uniformly reduce the intensity
of light over a broad range of wavelengths
Polarizers: Block or transmit light according to its polarization.
Often used to reduce glare.
Dual Bandpass filters: Dual
BP filters transmit all wavelengths within two separate wavelength ranges,
and block all others including those bnetween the two bands. This is often
used in traffic applications.
Issues to consider
• Problems with filters are more likely to be the result of using
the wrong filters
• Filters
degrade overtime, so they have to be changed - their life is finite
•
Absorption results in conversion of light into heat
and laser beams hitting colour glass filters may destroy the filters
• Use the least number of filters necessary
to reduce signal loss
• High-quality filters are a better investment than cheaper ones
• The wavelength at which
Dichroic filters transmit light shifts slightly with the angle at which
light hits the filter
• The wavelength at which Dichroic filters transmit light shifts
slightly with temperature. This is important to consider for narrow bandpass
filters.
| Model |
Colour
|
Passes
|
Absorbs
|
Reflects
|
Applications and
comments
|
| Colour Bandpass | |||||
| BP470 | Blue | 435-490nm | UV 270-380nm | most visible and IR | Recommended for use with 470nm blue LEDs and for UV fluorescence imaging |
| BP505 | Cyan | 475-565nm | UV and deep blue | 570-1100nm | For use with 500-510nm cyan LED lighting. Also has a function in UV fluorescence imaging of some commonly-used stains |
| BP525 | Light green | 495-565nm | UV and blue | 570-1100nm | Recommended for use with green LEDs and fluorescence imaging |
| BP590 | Orange | 555-615nm | 200-550nm | 620-1090nm | Recommended for use with amber/orange LEDs and fluorescence imaging |
| BP635 | Light Red | 600-660nm | 200-590nm | 670-1080nm | Recommended for use with red laser diodes and LEDs |
| BP660 | Dark Red | 635-690nm | 200-630nm | 695-1100nm | Recommended for use with dark red LEDs |
| Model |
Colour
|
Passes
|
Absorbs
|
Reflects
|
Applications and
comments
|
| UV Bandpass | |||||
| BP250 | Broadband UV | 290-380nm |
N/A
|
390-1200nm | Requires camera with deep-UV sensitivity and lens designed specifically for UV imaging |
| BP324 | UV-A/B* | (UV) 270-375nm and (NIR) 690-750nm | Visible and most IR |
N/A
|
*UV-A = 400-320nm; UV-B = 320-290nm; UV-C = 290-100nm. Particularly useful in true UV imaging applications, where blocking longer wavelengths is essential |
| BP365 | Blacklite/UV-A | UV 315-400nm, 815nm and above | N/A | visible light and near-IR: 410-805nm | Blocks visible light at the source while passing UV. Good for filtering visible light from mercury and metal halide lighting. |
| Model |
Colour
|
Transmission range
|
Applications and
comments
|
||
| Colour Narrow Bandpass | |||||
| BN470 | Narrow Blue | UV Block 450-495 nm | • Narrow design for overwhelming
ambient light conditions • Suitable for laser diodes • Commonly used in fluorescence applications • Less sensitive to angle of incidence variations than traditional interference filters |
||
| BN532 | Narrow Green | 515-560 nm | |||
| BN595 | Narrow Orange | 570-615 nm | |||
| BN630 | Narrow Light Red | 610-650 nm | |||
| BN660 | Narrow Dark Red | 640-680 nm | |||
| Model |
|
Transmission range
|
Applications and
comments
|
||
| Infrared Narrow Bandpass | • Narrow design for overwhelming
ambient light conditions • Types offered for all wavelengths commonly found in IR illuminations • Ideal for security, traffic control and industrial inspection applications. • Suitable for laser diodes • Less sensitive to angle of incidence variations than traditional interference filters |
||||
| BN740 | 720-770 nm | ||||
| BN785 | 760-805 nm | ||||
| BN850 | 825-870 nm | ||||
| Model |
|
Transmission range
|
Applications and
comments
|
||
| Dual Bandpass | • Colour camera day/night applications • Blocks interfering IR wavelength range to achieve accurate colour rendition • Allows the ability to view a scene with appropriate IR illumination at night |
||||
| DB850 | Visible + 850 nm IR Visible 385-650 nm Infrared 820-880 nm | ||||
| DB940 | Visible + 940 nm IR Visible 385-650 nm Infrared 910-970 nm | ||||
| Model |
|
Passes
|
Absorbs
|
Reflects
|
Applications and
comments
|
| Infrared Pass - Visible Block | |||||
| LP695 | IR Longpass | 700nm and above | 690nm and below |
N/A
|
Used with monochrome cameras to aid in increased contrast and resolution |
| LP780 | IR Longpass | 790nm and above | 775nm and below |
N/A
|
For structured laser diode modules and LED lighting that operate in the range around 810-830nm |
| LP830 | IR Longpass | 830nm and above | 815nm and below |
N/A
|
90% of all applications is either 880nm or 850nm LED lighting. Recommend for 880nm when the choice of IR lighting will be made at a later time |
| LP920 | IR Dichroic longpass | 920nm and above | 690nm and below | 690-910nm | Lower wavelength blocking allows these filters to block visible emissions (particularly the red "glow") when placed in front of multi-spectral lighting |
| LP1000 | IR Longpass | 1010nm and above | 980nm and below |
N/A
|
Used with infrared vidicon or InGaAs cameras, where it cuts off wavelengths below the 1550nm telecommunications band |
| BP695 | IR Bandpass NIR fluorescence |
670-715nm | 660nm and below | 720-1100nm | Designed to function in the 680-715nm range such as with 700nm LED lighting. particularly useful for imaging typically weak luminescence in the 680-710nm range where the subject is often excited with 635nm or 660nm red LED lighting. |
| BP735 | IR Bandpass IR LEDs @ 735nm |
700-770nm | 690nm and below | 780-1065nm | Particularly useful for imaging typically weak luminescence in the 715-765nm range |
| BP800 | IR Bandpass | 730-1030nm | 720nm and below, 1050nm and above |
N/A
|
Best choice if thin filter needed over sensor. Recommended for use with near-infrared lighting. Can be used in conjunction with several commonly- used LED and laser diode wavelengths. |
| BP850 | IR Bandpass High-power IR LEDs @ 850nm |
IR 810-990nm | 200-800nm | 1010nm and above | Recommended for use with high-power IR LEDs |
| BP880 | IR Bandpass IR LEDs @ 880nm |
IR 840-1010nm | 200-840nm | 1010nm and above | Recommended for use with high-power 880nm IR LEDs. Significantly improves and maximizes contrast when working in the infrared |
| Model |
|
Passes
|
Absorbs
|
Reflects
|
Applications and
comments
|
| Infrared Block - Visible Pass | |||||
| BP550 | NIR+UV Block/Visible Pass | 410-690nm |
N/A
|
350-400nm and 710-1550nm |
For all colour applications where filtering is not provided with the camera's sensor |
| SP645 | Mid-Red/NIR Dichroic Block | 380-640nm |
N/A
|
650-1065nm | Best IR/mid-red block |
| SP675 | Deep Red/NIR Dichrioc Block | 380-670nm |
N/A
|
680-1080nm | Best IR/deep red |
| SP700 | Hot Mirror/NIR Dichroic Block | 410-690nm |
N/A
|
350-400nm and 710-1550nm |
Standard IR block, for use with monochrome cameras |
| SP730 | Hot Mirror/NIR Colourless Block | 370-725nm |
N/A
|
735-1100nm | Standard IR blocking, for use with colour cameras |
| Model |
|
Passes
|
Absorbs
|
Reflects
|
Applications and
comments
|
| Polarizing | |||||
| PR032 | Rotating Linear Polarizer | visible (340nm+) and IR | UV 200-340nm |
N/A
|
Used to eliminate glare in the visible spectrum. Note: The polarization of light only takes place over the 400-700nm (visible) range; light in the UV or IR is not polarized. |
| PC052 | Circular Polarizer | visible (340nm+) and IR | UV 200-340nm |
N/A
|
Reduces glare; for autofocus or autoiris lenses; for visible spectrum applications |
| PI035 | IR Linear Polarizer | Visible (340nm+) and IR, or only IR (two types available) | UV 200-340nm |
N/A
|
Reduces only infrared glare. Do not work in the near-infrared. |
| Model |
|
Passes
|
Absorbs
|
Reflects
|
Applications and
comments
|
| Neutral density - absorptive | |||||
| ND030 | 380nm and above |
N/A
|
N/A
|
Reduce luminous intensity without affecting colour. Optical Density = 0.3 (T = 50%). Sometimes referred to as "2X" filters | |
| ND060 | 389nm and above |
N/A
|
N/A
|
Optical Density = 0.6 (T = 25%) – reduces light intensity. Sometimes referred to as "4X" filters | |
| ND090 | 390 and above |
N/A
|
N/A
|
Optical Density = 0.9 (T = 12.5%) – reduces light intensity. Sometimes referred to as "8X" filters | |
| ND120 | 395nm and above |
N/A
|
N/A
|
Optical Density = 1.2 (T = 6.3%) – for viewing into lamps. Sometimes referred to as "16X" filters | |
| Neutral density - low reflectivity | |||||
| NI030 | 350nm and above |
N/A
|
N/A
|
Optical Density = 0.3 (T = 50%) Visible and Near-IR Imaging. Sometimes referred to as "2X" filters | |
| NI060 | 350nm and above |
N/A
|
N/A
|
Optical Density = 0.6 (T = 25%) Visible and Near-IR Imaging. Sometimes referred to as "4X" filters | |
| NI090 | 350nm and above |
N/A
|
N/A
|
Optical Density = 0.9 (T = 12.5%) Visible and Near-IR Imaging. Sometimes referred to as "8X" filters | |
| NI0120 | 350nm and above |
N/A
|
N/A
|
Optical Density = 1.2 (T = 6.25%) Visible and Near-IR Imaging. Sometimes referred to as "16X" filters. For viewing into lamps, furnaces, welding processes, etc. | |
| Model |
|
Passes
|
Absorbs
|
Reflects
|
Applications and
comments
|
| Longpass Colour | |||||
| LP500 | Yellow | 500nm and above | 480nm and below |
N/A
|
Blocks most blue and UV wavelengths used with mono cameras for improving contrast, resolution and separating colours in black/white or colour applications. Typically not recommended for machine vision unless blocking of multiple wavelengths is necessary. |
| LP515 | Yellow-Orange | 510nm and above | 500nm and below |
N/A
|
Blocks all blue and UV while passing all other wavelengths. Typically not recommended for machine vision unless blocking of multiple wavelengths is necessary. |
| Model |
|
Passes
|
Absorbs
|
Reflects
|
Applications and
comments
|
| Shortpass/Notch Colour | |||||
| SP510 | Blue Dichroic | 350-500nm, 810nm and above |
N/A
|
510-800nm | For sorting gold from silver or plated metal parts |
| SP570 | Blue-Green Dichroic | visible, 400-570nm |
N/A
|
500-1060nm | Blocks orange to NIR |
| SP585 | Cyan Dichroic | 370-580nm, 830nm and above |
N/A
|
590-820nm | Blocks red and very near IR |
| SP625 | Blue-Orange Dichroic |
N/A
|
Blocks deep red/NIR | ||
| NF550 | Magenta Dichroic (Green Block) | 320-465nm, 610nm and above |
N/A
|
470-600nm | Blocks green while passing blue and red wavelengths. More commonly referred to as "Notch Filters" |
| Model |
|
Passes
|
Absorbs
|
Reflects
|
Applications and
comments
|
| White light balancing | |||||
| LA120 | Minus Blue + | 500nm+ | UV and blue 200-500nm | 400-500nm | Removes blue cast from white LED and metal halide lighting. Preferred for use with white LED lighting when accurate colour rendering is desired. |
| LB080 | Minus Red | 370-520nm | 360nm and below; 540-1100nm |
N/A
|
Removes yellow cast from halogen lighting. Often used in association with fibre optic lighting. |
| FL550 | Minus Green | 400nm and above | 390nm and below |
N/A
|
Removes green cast from fluorescent. Absorbs somewhat in the middle of the visible portion of the spectrum, which is also where some colour cameras have their strongest visible response, resulting in improved colour rendition. |
| Model |
|
Passes
|
Absorbs
|
Reflects
|
Applications and
comments
|
| Acrylic Filters | |||||
| AC380 | Useful range: 450 - 850nm. A/R Acrylic Protective Window – abrasion, scratch, breakage- and solvent-resistant acrylic window for regulated applications where glass can't be used | ||||
| AC685 | Useful range: 700 - 1100nm. Acrylic NIR Longpass – allows through mainly IR light | ||||
| AC760 | Useful range: 450 - 850nm. Acrylic NIR Longpass – allows through mainly IR light and also blocks very near IR | ||||
What is Optical Density
(OD)?
Optical density is a measure of how much light is blocked by a filter.
Optical Density-OD = -log10 (T).
A filter with 1% transmission has an OD of 2
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