Properties and Applications of Infra Red Linear Variable Filters

 

Linear Variable Filters have been used for many years in the visible and IR  (ref 1 & 2) and are gradually gaining popularity in optical systems. There is an increasing demand for  measuring optical properties ‘in the field’. This can save valuable amounts of time and resources, the instruments must be compact, portable , and easy to use. This demand has led to a drive for miniaturization and an LVF can play a key role in this. The Infra Red spectrums of many gases, chemicals and industrially manufactured materials, can be used as the means to positively identify and monitor them.

 

 

2.0 DESCRIPTION AND PROPERTIES OF AN LVF.

 

The coatings on an LVF are of standard optical design, the key difference being that the coating thickness varies in a fixed and regular way along one direction. Standard deposition techniques can be used to deposit these layers. Fig 1 below is a schematic for a 2500-5000nm LVF, the coating behaves as a narrow band filter with the centre wavelength (CWL) changing from 2500-5000 nm along its length, the width of the filter (FWHM) is approx 1.5% of the CWL.

 

Fig 1-Basic Layout of a 2500-5000nm  Linear Variable Filter.

 

 

 

The filters are environmentally durable and pass the standard MIL spec. tests expected of such coatings, for example, adhesion, humidity, severe abrasion, cleaning with solvents and salt solution exposure as per MIL-C -48497.  The narrow band filters also have a temp stability of <0.1 nm/deg C.

 

 2.1 Optical Properties of a 900-1700 nm LVF

 

The design and manufacturing  processes can be ‘shifted’ into different spectral regions, Fig 2 below shows a more detailed performance for an LVF for 900-1700 nm. The transmission is shown at regular intervals along its length. The blocking outside the transmission zone  averages OD<3 and extends from the UV to beyond 1800 nm.

 

Fig 2: Optical Performance of Actual LVF for 900-1700 nm.

 

 

 

 

3.0 POTENTIAL APPLICATIONS OF IR LVFs

 

The LVFs can be produced in typical sizes around 15 mm x 3.5 mm wide x 0.5 mm thick for example. The LVF for 900-1700nm  above could  be positioned in front of an  InGaAs line array detector, this could be the basis of a scanning spectrophotometer  for use in the processing of  many industrial materials with an IR fingerprint in this region. The robust nature of the coatings mean they are suitable to work in potentially harsh environments such as factories, chemical plants and  high and low temperatures.  Hyperspectral Imaging applications for space programs are worth considering, the potential to make an instrument more compact and reduce payload size and cost is one application currently being assessed. A 2500-5000 nm LVF with a lead salt linear array detector  could be used for gas detection in the mid IR .

 

4.0 POTENTIAL FOR VOLUME PRODUCTION OF LVF S

 

Large volume production is possible for the current deposition technology,  the deposition process will scale up. The unit price, presently in hundreds of euros per filter, would drop to less than 10-20 euros  per filter. Such a price drop in the years ahead will increase the applications but this would also need to be matched by a commensurate drop in line array detector prices.

 

REFERENCES

 

 [1] A. Emadi et al, IR Microspectrometers based on Linear-Variable Optical Filters, University of Technology, Delft, Netherlands, Proc. Eurosensors XXV, September 4-7, Athens Greece.

[2] Dr Oliver Pust, Innovative Filter Solutions for Hyperspectral Imaging, Optik & Photonik Volume 11, Issue 3, pages 24-27, June 2016

 

 

KEY WORDS 

IR Linear Variable Filters; Mini Spectrophotometers ; Hyperspectral