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Optical filters
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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.

MIDOPT Optical Filters: select by type
• Colour Bandpass • UV Bandpass • Colour Narrow Bandpass • Infrared Narrow Bandpass • Dual Bandpass
• Infrared Pass - Visible Block • Infrared Block - Visible Pass • Longpass - Colour • Neutral Density • Polarizing
• Shortpass / Notch Colour • White-Light Balancing   --- FILTER MOUNTS ---
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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