SELFOC® Imaging Lenses

Advantages:

 

  Small Diameter
  Stable Cylindrical Shape
  Simplified Mounting

 

 
The SELFOC Imaging lens is commonly used as an objective lens for small diameter imaging systems where conventional lenses are not suitable due to size limitation. The lens is designed to gather light from an object and form an inverted image at the back surface of the lens. Typical applications include fiberscope (attached to a coherent fiber bundle) and rigid endoscope (attached to a relay lens such as a SELFOC Rod Lens, see next section).

SELFOC Imaging Lenses are specified somewhat differently that SELFOC Micro Lenses (SML). While SML performance is controlled through the lens pitch and gradient constant (√A), Imaging Lens performance is controlled via the working distance (WD) and image quality factors such as resolution and field curvature. For this reason, ILW and ILH lenses can consistently exhibit good image quality despite variations in √A and/or Z (Lens Length).

SELFOC® Imaging Lenses - Technical Chart

Lens Type & Dia.(mm)Lens Length Z (mm)Resolution (LP/mm) centerResolution (LP/mm) @0.8RTyp. Chromatic Aberration (µm)Typical √A (mm-1)Magnif-1 at WD=5 mmField Curvature (μm)View Angle (2θ)Refract. Index (N0)
ILW-0.250.70 +/- 0.104001502.31919.14050° min.1.643 on axis
ILW-0.350.96 +/- 0.10300120491.7114.140
ILW-0.501.39 +/- 0.13250100511.2039.950
ILW-0.702.00 +/- 0.2020080820.8567.160
ILW-1.002.95 +/- 0.30200501020.65.180
ILW-1.303.96 +/- 0.35180401440.4623.9100
ILW-2.006.54 +/- 0.60160302610.2992.7150
ILW-2.709.34 +/- 0.80140203140.2222.1200
ILH-0.250.50 +/- 0.104001503.12626.94070° min.1.666 on axis
ILH-0.350.74 +/- 0.10350120352.19618.460
ILH-0.501.05 +/- 0.1530080421.56913.180
ILH-0.701.50 +/- 0.2028060471.1189.4100
ILH-1.002.19 +/- 0.3025030760.7866.7100

Specification Notes:

cat14

  • Standard working distance is 5 mm for all ILW and ILH.
  • Diameter tolerance is +0/-0.05 mm for all ILW and ILH.
  • Resolution is measured on a U.S. Air Force chart while a 100X microscope is focused on the back surface of the imaging lens (see Figure). The chart is placed at 5 mm, the standard working distance, away from the lens. Resolution is measured at both lens center and 80% of the lens radius and normalized by the magnification factor.
  • Field curvature is the difference in focal positions between the lens center and at 80% of the lens radius.
  • End surface is inspected at 20X magnification. No chips and cracks are allowed within 90% of the lens radius.
  • Material Toxicity: This product contains components which might be toxic. The user is advised to pay special attention to such toxicity when using this product in medical devices for people or animals.

 

ILW-01-01-01-01

 
Physics of SELFOC

The Gradient Constant

The SELFOC lens utilizes a radial index gradient. The index of refraction is highest in the center of the lens and decreases with radial distance from the axis. The following equation describes the refractive index distribution of a SELFOC lens:

Equation 1: 
N(r) = N0(1 – ((√A2)/ 2 * r2)

This equation shows that the index falls quadratically as a function of radial distance. The resulting parabolic index distribution has a steepness that is determined by the value of the gradient constant, √A. Although the value of this parameter must be determined through indirect measurement techniques, it is a characterization of the lens’ optical performance. How rapidly rays will converge to a point for any particular wavelength depends on the gradient constant. The dependence of √A and N0, on wavelength is described by the dispersion equations listed at the end of this product guide. Note that different dispersion equations apply to different lens diameters and numerical apertures.

Lens Length & Pitch

In a SELFOC lens, rays follow sinusoidal paths until reaching the back surface of the lens. A light ray that has traversed one pitch has traversed one cycle of the sinusoidal wave that characterizes that lens. Viewed in this way, the pitch is the spatial frequency of the ray trajectory.

Equation 2:
2πP=(√A)*Z

The above equation relates the pitch (P) to the mechanical length of the lens (Z) and the gradient constant. The figure below illustrates different ray trajectories for lenses of various pitch. Notice how an image may be formed on the back surface of the lens if the pitch is chosen appropriately.

Paraxial Optics

In contrast to the optics of homogeneous materials, gradient-index optics involve smoothly-varying ray trajectories within the GRIN media. The paraxial (first-order) behavior of these materials is modeled by assuming sinusoidal ray paths within the lens and by allowing the quadratic term in Equation 1 to vanish in the ray-tracing calculations. All of the usual paraxial quantities may be calculated with the help of the ray-trace matrices given at the end of this product guide. The formulae for common paraxial distances have also been tabulated for quick reference.

 

Recommended Storage and Handling of Lenses

Storage
For extended periods of time, the lenses should be stored in a “dry box” environment (40%RH or less). This entails the use of a desiccant (e.g., silica gel) or a heat source to prevent humidity from leaching the lens material. This is much more critical for non-coated lenses, since AR coatings help to protect the lens surfaces from humidity. For short term storage (less than a month), the plastic box and foam packing in which the lenses are shipped will provide adequate storage.

In addition to humidity requirement, the lenses need to have sufficient spacing to avoid potential damage such as chipping and scratching from other lenses. For this reason, Go!Foton storage boxes have built-in slots in which the lenses are placed, with surrounding packaging to hold them securely in place.

Handling
After opening the lens boxes, it is important to exercise extra care in lifting the plastic shield. Particularly with smaller lenses, it is possible that they may cling to the shield and be lost during removal. Lenses should be handled with plastic tweezers, preferably those with a tapered end. Lenses should be picked up out of their individual compartments by firmly holding each by its side surface (not the ends).

Cleaning
At times it is necessary to clean the lens surfaces due to the presence of some dust or film which may impair the image. Go!Foton generally recommends the use of ethyl alcohol as a cleaning solvent. Acetone may also be used, without harm to the lens, but it should be pure enough to no leave a residue on the lens’ surface.