Preface: Basic Concepts of F-Number and Depth of Field

1. What is F-Number?

F-number (F/#, aperture value) is a core parameter of optical lenses, defined as the ratio of the lens focal length to the effective clear aperture diameter (F/# = Focal Length / Aperture Diameter). It directly determines the light-collecting ability of the lens—the smaller the F-number, the larger the aperture diameter, and the more infrared energy reaches the detector, resulting in higher thermal sensitivity (NETD) of the system. In infrared technology, the F-number not only affects image brightness but is also critical to diffraction limits and compatibility with the detector.

2. What is Depth of Field?

Depth of Field (DOF) refers to the distance range in front of and behind the focal plane within which objects appear acceptably sharp. It determines the allowable deviation of a target from the precise focus point along the depth direction. DOF is jointly influenced by three factors: F-number, focal length, and focus distance. Specifically, the larger the F-number (smaller aperture), the greater the DOF; the shorter the focal length, the greater the DOF; the farther the focus distance, the greater the DOF.

3. The Relationship Between F-Number and Depth of Field:

While determining light throughput, the F-number also directly regulates the depth of field range. There is an inherent trade-off between the two:

• Small F-number (Large Aperture): High light throughput, beneficial for improving imaging quality under low-signal conditions, but results in shallower DOF, requiring higher focus accuracy.
• Large F-number (Small Aperture): Lower light throughput, but provides greater DOF, maintaining image sharpness over a wider depth range.

In the design of MWIR and LWIR lenses, this trade-off is particularly crucial—the differing requirements for sensitivity and DOF between cooled and uncooled detectors dictate the strategy for F-number selection.

F-Number & Depth of Field Technical Q&A

I. Core Q&A on F-Number (Aperture Value)

1. Q: Why do cooled MWIR lenses typically use a fixed aperture like F/2.0 or F/4.0, rather than an adjustable one?

A: Cooled MWIR detectors (e.g., InSb) contain an internal, very cold aperture stop called a cold stop. The lens aperture must be precisely matched to this cold stop to achieve 100% cold stop efficiency. If the aperture is adjustable or mismatched, the detector will receive thermal radiation from the lens housing, leading to inaccurate temperature measurements or "cold reflection" artifacts (central dark spots) in the image. Therefore, the aperture of cooled MWIR lenses is typically fixed, and the F-number must strictly follow the detector's design value (e.g., F/2 or F/4).

2. Q: Why do uncooled LWIR lenses typically pursue smaller F-numbers (e.g., F/1.0, F/1.1)?

A: Uncooled LWIR detectors (e.g., VOx or a-Si) have relatively lower sensitivity and lack an internal cold stop. To collect more infrared energy, thereby reducing the Noise Equivalent Temperature Difference (NETD) and improving image clarity and temperature measurement accuracy, the largest possible clear aperture is required. Consequently, uncooled LWIR lenses are typically designed with F/1.0 or even larger apertures. These are often called "fast lenses" as they achieve a better signal-to-noise ratio within the same integration time.

3. Q: Why is the F-number of LWIR lenses generally smaller than that of MWIR lenses?

A: The primary reason is that longer wavelengths are more susceptible to diffraction effects. According to the diffraction limit formula (Airy disk diameter ≈ 2.44λF/#), given the same pixel size, achieving a sharp image with longer wavelengths (LWIR, 8-14μm) necessitates a smaller F-number to constrain the Airy disk diameter and match the detector pixel size. For example, the diffraction spot size of an F/1.0 LWIR lens is comparable to that of an F/2.0 MWIR lens.

II. Core Q&A on Depth of Field Parameters

4. Q: What are the typical differences in Depth of Field between cooled MWIR lenses and uncooled LWIR lenses?

A: Cooled MWIR lenses typically have larger F-numbers (e.g., F/2.0 - F/4.0), and therefore inherently possess greater depth of field. This allows them to maintain sharp focus across the entire field of view more effectively when testing at close distances or observing targets with significant height variations. In contrast, uncooled LWIR lenses, due to their pursuit of large apertures (around F/1.0), have relatively shallower depth of field. This is particularly noticeable in macro or close-range applications, where inaccurate focusing can easily lead to partial blurring.

5. Q: How can image blur be avoided when observing targets at close distances?

A: First, it's essential to know the lens's minimum working distance. For uncooled LWIR cameras (e.g., equipped with an F/1.0 lens), the depth of field is very shallow at close distances, requiring precise focusing. If using a cooled MWIR camera with a larger F-number (e.g., F/4.0), its greater depth of field allows for a sharp full-field image even at close range. If your application primarily involves close-range work (e.g., circuit board inspection), choosing a lens with a slightly larger F-number or one with macro focusing capabilities would be more suitable.

6. Q: A smaller F-number results in shallower Depth of Field. How does this affect temperature measurement?

A: A shallower depth of field means that temperature measurement is most accurate only on the precisely focused plane. If the target object has significant depth variations (e.g., pipelines or complex mechanical components), or if the target deviates from the precise focus plane, even if the image doesn't appear drastically different, the distribution of infrared energy on the detector can change subtly. This might affect the temperature measurement accuracy for very small targets. For high-precision thermography, it is necessary to consider the lens's depth of field characteristics and ensure the measured target lies within the sharp depth of focus.

III. Typical Lens Parameter Comparison Table