Fluorescence endoscopy is becoming increasingly important in many medical diagnostic tests. Based on the molecular absorption of light, this technique works by revealing tissue abnormalities that are hidden from view under normal white light. Already, fluorescence endoscopy can improve the endoscopic detection of nonvisible malignant lesions or tumors, for example.

Sensitizers, chemicals that can cause allergic reactions, accumulate in these lesions. These sensitizers induce tissue fluorescence in response to certain wavelengths of light. In the field of gastroen-terology, this ability to reveal and visualize lesions holds promise for the early detection of dysplasia and cancers. Similarly, near-infrared (NIR) fluorescence rigid endoscopic imaging systems are emerging as a critical tool in some brain surgeries.

Fluorescence endoscopy, which is gaining ground in these and other medical applications compared to traditional white light endoscop-ic techniques, places some unique requirements on optical lenses (see the sidebar, “Current NIR Fluorescence Imaging Applications”). For example, a lens with the right combination of high-resolution, antireflec-tive coatings and minimal focus shift can enable better fluorescence imaging across the visible and NIR range, which includes wavelengths between 400 and 850 nm.

This article discusses various lens design considerations, including optical filters and antireflective coatings, that can further optimize the performance of NIR fluorescence endoscopy in medical and life science applications.

Antirefractive Lens-Coating Technologies

Recent advancements in coating and thin-filter optical technologies are resulting in more applications in the medical and life science industries, including optimizing NIR fluorescence endoscopic techniques. One such advancement, called the extended bandwidth and angular dependency (eBAND) lens coating, deploys a nanostructured layer with an ultra-low refractive index, the dimensions of which are smaller than the wavelengths of visible light.

This nanostructured layer, coupled with sophisticated multilayer coatings underneath, yields significant antireflec-tion properties. By suppressing tangential reflection, this coating also reduces undesired flare and ghosting to deliver sharp, clear images even in very poor lighting conditions.

Compared with other antireflective coatings, eBAND is suitable for wavelengths from 400 to 1,700 nm, which includes the visible to near-infrared range. This capability enables users of eBAND to conduct in-depth examinations with in-docyanine green (ICG), a medical dye frequently used in tests that involve the heart and liver, as well as certain parts of the eye.

High Optical Density

In addition to antireflective coatings, fluorescence endoscopy relies on lenses with highly precise optical filters, as well as lenses with a high optical density (OD). The purpose of these filters is to selectively transmit only certain wavelengths on the optical spectrum and reject others, while OD describes the amount of energy blocked by the filter.

A high OD indicates low energy transmission, while a low OD indicates high transmission. Optical densities of six or greater are typically used in applications that require extreme blocking like fluorescence endoscopy.

Fig. 1 - The full width at half maximum (FWHM).

Fluorescence endoscopy requires optical filters that enable precise control of both the laser beam incident angle and full-width-at-half-maximum (FWHM) value. The laser beam incident angle indicates the degree at which a laser beam hits the filter, whereas FWHM measures the optical bandwidth of a light source. FWHM is the distance between the points on a curve at which the function reaches half its maximum value — in other words, the width of the curve's bump. When applied to optics, it refers to the width of an optical signal at half its maximum intensity and provides the bandwidth of a light source operating at 50 percent capacity.

The ability to manage both the laser beam incident angle and FWHM results in highly accurate control of a light source, enabling affected areas in fluorescence endoscopy procedures to be seen more clearly (see Figure 1).

Minimal Focus Shift

Lenses for fluorescence endoscopy need to feature an optimized optical design that maintains image quality across a broad spectrum of wavelengths, all while minimizing the focus shift between the visible near infrared, and even shortwave infrared ranges (see the sidebar, “The Modulation Transfer Function” for more details).

The Modulation Transfer Function

Modulation transfer function (MTF) is a reference value that enables optical designers to quantify the overall imaging performance of a system in terms of its resolution and contrast. Designers often refer to MTF data in applications like fluorescence endoscopy that depend on imaging accuracy for success. Although a helpful value to know, MTF does not always reflect the real- world performance of the lens. Nonetheless, understanding the MTF curves of the lens and other parts within an optical system allows designers to select and optimize the components for a particular resolution.

For example, during testing, one Tamron lens eliminated the need to refocus between the visible light range and the NIR range (from 400 to 850 nm), as well as between the visible light range and the short-wave infrared range (from 400 to 1,700 nm).

Fig. 2 - The focus shift with a Tamron lens across the visible and NIR light range.

This lens minimizes the focus shift thanks to its optical design, which includes a variety of materials and spherical and aspherical lenses designed to meet the demands of various markets (see Figure 2).

Extra-Low Dispersion Glass

Fig. 3 - ODS-level filters with low-film thickness.

In imaging, chromatic aberration occurs when a lens element refracts different wavelengths at slightly different angles, causing color fringing and reducing the sharpness of an image. Low-dispersion (LD) lens elements, which are made of a special optical glass with extremely low dispersion indices, can combat this effect.

In other words, for this type of glass, the refraction of a ray of light into its colors is extremely narrow. As a result, these lens elements compensate for chromatic aberrations at the center of the field. They also compensate for lateral chromatic aberrations that can occur at short focal lengths toward the edges of the field (see Figure 3).

Fig. 4 - Comparing chromatic focus shifts between Tamron and conventional lens designs across visible and NIR ranges.

Similarly, anomalous dispersion (AD) optical glass offers even greater, more precise control of chromatic aberrations to enhance overall imaging performance. This type of glass provides an abnormally large partial dispersion ratio for specific wavelengths within the visible spectrum. Therefore, combining AD glass with normal glass makes it possible to control the dispersion factors of specific wavelengths. Optical lenses used in fluorescence endoscopy should avoid chromatic aberrations, maintaining focus shift consistency across extended wavelengths (see Figure 4).

This article was written by Kota Yamamoto, Section Manager, Medical Device Development, Tamron, Saitama, Japan. For more information, visit here .