UV and IR Wavelength Selection White Paper

UV and IR Wavelength Selection

A Component Engineering White Paper — wavelength, bandwidth and binning as yield-critical parameters in curing, disinfection and sensing systems

Queendom LEDs · Component Engineering Group

1. Why Wavelength Is a Yield Parameter, Not a Comfort Parameter

In visible lighting, a few nanometres of wavelength drift is largely a colour question. In UV curing, UV disinfection and IR sensing, it is a yield question. A photoinitiator absorbs in a narrow band; a microbial action spectrum peaks at a specific wavelength; a sensor’s filter passband is a few tens of nanometres wide. Selecting the emitter peak and spectral width is therefore a process decision with a measurable cost when it is wrong. This paper explains how to select UV and IR wavelength and bandwidth so that the system, not the component alone, meets its target.

It complements three application notes in our resource center: the UVC 275 nm disinfection dosage note, the UVA curing and sensing design note, and the SWIR wavelength selection for food sorting note.

2. How Wavelength and Bandwidth Are Specified

Three quantities govern whether an emitter will work in the target process: the peak wavelength, the full width at half maximum (FWHM), and the binning tolerance on both. Buyers who specify only the peak wavelength routinely discover that the delivered parts span a band wide enough to split the process.

It is useful to picture the emission spectrum as a shape rather than a number. The peak tells you where the shape is centred; the FWHM tells you how broad it is; the bin tolerance tells you how far the shape may move between production batches. A process that depends on a narrow chemical absorption band cares about all three, because energy emitted outside the absorption band is paid for but not used. This is why two emitters with identical peak wavelength and identical radiant flux can produce measurably different process results.

3. Selecting in the UVC Band

Disinfection applications are anchored near 265–280 nm. Within that band the choice is narrower than it appears: 275 nm is the common engineering compromise between germicidal efficacy and optical extraction, and it is the wavelength at which Queendom UVC components are binned. Note that 275 nm (J-15 series) and 254 nm (Z-14 low-pressure lamp reference) are distinct references and must not be treated as interchangeable when calculating dose.

4. Selecting in the UVA Band for Curing

UVA curing is a matching problem between the emitter and the photoinitiator, not a power problem. A 395 nm emitter and a 365 nm-sensitive adhesive can both be called ‘UVA’ while producing very different cure depths. The correct method is to read the adhesive’s absorption spectrum, select the emitter peak to fall within it, and then size the irradiance from the required dose.

5. Selecting in the IR and SWIR Bands

IR selection is dominated by the detector and the environment rather than by a chemical response. For illumination and machine vision, the emitter must sit inside the camera’s response and outside the dominant ambient noise. For SWIR material sensing, the emitter must sit on a characteristic absorption feature of the material being sorted.

5.1 IR illumination and machine vision

850 nm and 940 nm are the two workhorses. 850 nm offers more silicon camera response at the cost of a faint visible red glow; 940 nm is nearly invisible and is preferred for discreet illumination, but demands a camera with adequate sensitivity. The choice is made by the camera, not by the emitter.

5.2 SWIR for sorting and material identification

SWIR emitters are selected against the absorption spectrum of the target. Moisture absorbs near 1450 nm; many plastics have distinctive features between 1100 and 1700 nm. The emitter bandwidth must be narrow enough not to average across the feature, otherwise the contrast that makes sorting possible is washed out.

6. Temperature and Current: The Two Hidden Variables

Most wavelength selection is done at one temperature and one current, then expected to hold in the field. It does not. Both the peak wavelength and the FWHM move with junction temperature and with drive current, and the movements are large enough to matter in narrow-band processes.

7. Common Mistakes and How to Avoid Them

8. How to Write the Specification

Wavelength requirements are frequently under-specified because the process is described in words rather than in numbers. The following fields make the requirement measurable and therefore testable at goods-in.

8. Worked Example: UV Curing Cell for an Adhesive Bond Line

A production cell must cure a 1 mm adhesive bead on an opaque black housing. The adhesive datasheet lists a peak absorption near 385 nm and a required dose of 1.5 J/cm² at the bond line. The selection proceeds as follows.

The instructive part of this example is that the drive current is not the primary variable. A higher current that shifts the peak away from 385 nm can reduce cure quality even while increasing measured power. Matching the spectrum and then sizing the irradiance is the reliable order of operations.

10. A Wavelength Selection Checklist

12. Conclusion

Wavelength selection in UV and IR is a matching discipline. The question is never ‘which wavelength is best’, but ‘which wavelength and bandwidth match this chemistry, this detector and this environment, within the tolerance the process can absorb’. Specifying peak, FWHM, bin tolerance and shift together is what separates a process that works in the lab from one that works in production.

Download the full white paper

This white paper is published as part of the Queendom LED technical library. For datasheets, test reports and application notes referenced above, see the LED Components Support and Resource Center sections. Engineering enquiries: sales@queendomlamp.com.