Bandpass, Longpass, Shortpass - Can't Tell Them Apart? Everything Users Need to Know About Infrared Filter Selection

Bandpass, Longpass, Shortpass - Can't Tell Them Apart? Everything You Need to Know About Infrared Filter Selection

A purchasing specialist who was relatively new to the field once told us a story. Their team was developing an infrared temperature measurement module. They selected a longpass filter. During debugging, they found an anomalous response to short-wave interference. Two weeks of checking code later, they finally traced the problem back to the filter. What they actually needed was a bandpass, not a longpass.

Two weeks.

This type of mistake happens more often in this industry than most people are willing to admit. The reason is simple: bandpass, longpass, and shortpass sound somewhat similar, but their functional differences are fundamental. Get it wrong at the selection stage, and the debugging cycle pays the price.

This article explains the logic behind all three types, and answers a few of the most frequently asked questions during filter selection.

What Is a Longpass Filter, and When Do You Use It?

A longpass filter transmits only light with wavelengths longer than a certain cutoff point. All light with wavelengths shorter than the cutoff is blocked.

Here is a concrete example: a longpass filter with a cutoff wavelength of 700nm blocks all visible light below 700nm, while transmitting near-infrared and mid-to-far-infrared light above 700nm.

Image: https://ecdn6.globalso.com/upload/m/image_other/2026-07/4932/image-8.png

This type of filter is suitable for applications where you need to block all short-wave light and let longer wavelengths pass freely. The most typical use case is at the front end of thermal imaging systems - the 8-14m long-wave infrared needs to reach the detector, while visible light and near-infrared are entirely unwanted. A single longpass filter solves the problem with a simple structure.

MULTI IR's longpass series covers multiple cutoff wavelength nodes from near-infrared to mid-and-far-infrared, with stable shipments in thermal imaging, pyrometers, fire detection, and other applications.

However, longpass has a clear limitation: it only blocks short wavelengths. It does not block long wavelengths. If your system has interference from wavelengths longer than your target band, the longpass filter cannot address it. That is where bandpass comes in.

Image: https://ecdn6.globalso.com/upload/m/image_other/2026-07/4932/image-9.png

What Is a Shortpass Filter? Its Use Cases Are Fewer Than You Might Think

A shortpass filter does the opposite of a longpass - it transmits only light with wavelengths shorter than the cutoff point, while blocking all longer wavelengths.

Image: https://ecdn6.globalso.com/upload/m/image_other/2026-07/4932/image-10.png

In the infrared domain, shortpass filters are used significantly less frequently than longpass or bandpass. The most common scenario is when a system needs to completely isolate infrared radiation above a certain wavelength while preserving signal in the short-wave band. For example, certain visible-light camera systems that require near-infrared rejection use shortpass filters for blocking.

The core value of a shortpass is that it blocks long wavelengths. If your requirement is to eliminate all light above a specific wavelength, shortpass is the most straightforward choice.

What Is a Bandpass Filter, and Why Is It the Most Widely Used Type in Infrared Sensing Applications?

A bandpass filter has both a lower and an upper cutoff edge - it transmits only light within a specific wavelength range, while blocking both shorter and longer wavelengths.

This "specific range" is its transmission band. The transmission band is described by its starting and ending positions. For example, a transmission band from 3000nm to 5000nm, with all shorter and longer wavelengths blocked, is a bandpass filter. In the industry, products with a bandwidth-to-center-wavelength ratio greater than 5%-20% are often referred to as broadband filters. For instance, a bandpass filter centered at 850nm with a half-bandwidth of 40nm allows roughly only the 830-870nm range to pass, while blocking everything else.

Image: https://ecdn6.globalso.com/upload/m/image_other/2026-07/4932/image-11.png

Bandpass filters are the most widely used in infrared sensing because the vast majority of sensing applications share one common requirement: only the signal from the target wavelength band is needed - everything else is rejected.

Gas detection requires only the target gas absorption peak. Near-infrared facial recognition requires only the emission band of the illuminator. Laser ranging requires the specific emission wavelength of the laser. Time-of-flight ranging requires the narrow band corresponding to the transmitted pulse. None of these scenarios are satisfied by "blocking just one side." They require both sides to be blocked, leaving only the target band.

Is a Narrowband Filter a Separate Category? How Does It Relate to Bandpass?

A narrow bandpass filter is fundamentally a subset of bandpass filters. The only difference is the width of the transmission band.

In the industry, products with a bandwidth-to-center-wavelength ratio of less than 1%-5% are generally referred to as narrowband filters. However, this boundary is not an industry standard - definitions of "narrowband" vary between suppliers. When selecting, always look at the specific FWHM (Full Width at Half Maximum) value, not just the name.

Narrowband filters are used in applications that demand extremely high wavelength purity: channel isolation in laser communication, characteristic absorption peak selection in gas detection, and excitation and emission wavelength separation in fluorescence detection. In these scenarios, letting just a few extra nanometers of out-of-band light in would contaminate the signal. Narrowband is the only viable option.

Image: https://ecdn6.globalso.com/upload/m/image_other/2026-07/4932/image-12.png

MULTI IR's infrared narrowband filters are manufactured with center wavelength tolerance within plus-minus 2nm - a precision level that has been validated in gas detection and laser applications.

How to Quickly Determine Which Type You Need

Ask yourself three questions - that is enough.

1. Do I need to block one side or both sides?

If you only need to block short wavelengths, choose longpass. If you only need to block long wavelengths, choose shortpass. If you need to block both sides, choose bandpass.

2. How wide is my target signal band?

If your target band is very broad (for example, the entire mid-infrared 8-14m range), broadband or longpass may be sufficient. If your target band is very narrow (for example, the CO 4.26m absorption peak), narrowband is required.

3. Are there interference sources on either side of my target band?

If yes, bandpass is the only type that can address interference from both sides simultaneously - neither longpass nor shortpass can do that.

Once you have answered these three questions, the filter type is essentially determined. The remaining work, after confirming the type, is to define the specific center wavelength, bandwidth, and blocking depth parameters based on your system's source power, detector sensitivity, and target detection accuracy.

One Question That Comes Up Frequently:

For the same wavelength band, bandpass is more expensive than longpass - why? Where does the extra cost come from?

It comes from the complexity of the coating.

Image: https://ecdn6.globalso.com/upload/m/image_other/2026-07/4932/image-13.png

A longpass requires coating design on only one cutoff edge - relatively straightforward. A bandpass requires simultaneous control of the position and slope of two cutoff edges, with significantly more coating layers and higher demands on coating equipment stability and process control. The narrower the bandwidth, the more difficult the coating design - layer counts can reach dozens or even hundreds, and the requirements for the vacuum chamber environment approach demanding levels.

So when you see two filters with significantly different prices, do not jump to conclusions about one being overpriced. Check the type and bandwidth first - these two parameters alone explain most of the price difference.

Bandpass, longpass, and shortpass are not three siblings with similar-sounding names. They are three fundamentally different optical solutions for three different problems. Understand the boundaries of each type, and you will not go wrong in filter selection.

If you are still unsure which filter to choose, or if you have existing system parameters you would like to cross-check, please contact the MULTI IR technical team. We carry full-series stock covering bandpass, longpass, and shortpass - from near-infrared to mid-and-far-infrared - and support custom specifications with measured spectral curves.

MULTI IR Full-series infrared bandpass, narrowband, longpass, and shortpass filters in stock - covering the entire spectrum from near-infrared to mid-and-far-infrared. Over 10,000 types of infrared sensitive components available.Top 3 globally in comprehensive strength.National-level "Little Giant" Specialized and Sophisticated Enterprise.

Website: www.mirhz.com [https://www.mirhz.com/] Global Site: www.miroptech.com [https://www.miroptech.com/]

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