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yazhuge

2005-10-20 21:06

Lens design: then and now

Lens design: then (1964) and now (1996) by Bruce H. Walker

Much has been said and written about the changes to the lens design process over the recent decades. Having worked in this field for the past 30+ years, I have enjoyed being involved with many of those changes. This paper will share some general observations on the subject, and then quantify the impact of those changes by review of a specific lens design first executed in 1964 and then updated in 1996 by application of today's technology.

The lens design process
Typically, the lens design process begins with a set of optical specifications describing a lens that must meet an established set of performance criteria. Next, the lens designer calls upon personal experience, along with that of others, to identify an existing lens form that has the potential of meeting those specifications with a minimum of modification. This starting lens form is then manipulated by the lens designer, in an effort to make it conform to the established optical specifications. Key to this lens optimization stage are the tools used by the lens designer in the process. Typically, there are three basic tools involved: the computer (hardware), the lens design program (software), and the skill of the designer. All three are essentialno one is more important than the others. The final phase of the lens design process is the unambiguous documentation of the final lens prescription (suitable for manufacture), and of the optical performance that can be expected of the final design.

Hardware and software
While the experience and skill of the lens designer is difficult to quantify, improvements in hardware and software over the past 30 years have been vast, and are easily identified. Each lens designer who has worked in the field during these years has followed a path somewhat different from that of his or her colleagues. While typical may not be the best term, I believe my experience accurately reflects many of the changes that have occurred during this time period. Prior to establishing an engineering consulting business in 1991, I had worked for three companies, each involved in a unique aspect of the optics industry. Initially my lens design work was accomplished using a small (that meant it all fit into the same room) IBM-1620 computer, with a rudimentary software package provided by the computer manufacturer Later, I would work using a computer terminal, connected via phone lines, time sharing a large remote computer, with an optical design software package (ACCOS) installed on that computer. Around 1980 this changed to an arrangement where an in-house minicomputer was available, to be used with a leased software package (CODE IV). Today, I work with a Pentium PC, and have installed on that computer a reasonably priced lens design and optical engineering software package1, which meets all of my optical engineering and lens design requirements.

While attempts at cost and speed of computing comparisons are possible, I feel they have lost much of their relevance in recent years. Suffice it to say that the speed with which calculations are now executed far outstrips the designers ability to keep up with the volume of useful data output. Likewise with cost of computing, the speed with which solutions and data are generated, the bargain prices of today's hardware, and the cost of today's typical optical design software package, make the overall cost of computing (in most cases) trivial when compared with the fees being earned by the competent optical designer. Hats off to everyone involved in bringing about this spectacular revolution. Not only have they made all of us better designers and engineers, they have made our lives and work easier and a lot more fun.
A typical example
I would like to illustrate a few of the changes that have occurred over the past 30 years by taking a single lens design, one that I was responsible for in 1964, and describing how that design was generated. Recently, I have restored this design, evaluated its image quality, and examined the potential for its improvement by application of today's tools and techniques.

Early in 1964 I was presented with a request for a lens design that would meet the following specifications:

Effective focal length (EFL): 260 mm
Aperture: f/3.0
Image size: 25-mm dia. (5.5-deg field-of-view)
Wavelength: 1.06 µm (laser energy)
Overall length (First surface to image): 260 mm
Resolution: > 25 line pairs/mm
The aperture and field specifications indicated the need for a lens form generally referred to as the Petzval type. This form contains two widely spaced lens groups, with the distance to the image plane (back focus) equal to about one half the lens group separation. In order for the overall length to be approximately equal to the EFL, it is required that essentially all of the lens power be contained in the first lens group, while the second group has zero net power, but is used only to correct aberrations. A starting lens form with three elements in the first group and two in the second was chosen as a starting point for the design process.

The basic approach used was to first, optimize image quality on a curved surface, and then to add a field flattener lens as a final step. Being a single wavelength design, the choice of glass types was less critical than is generally the case. It was important to select a glass type that would be compatible with handling of laser energy. This glass would need to be essentially free from strafe, bubbles and surface blemishes.

Rather than working with exact ray trace data, the optical design software in use at that time would only permit the correction of 3rd order aberration coefficients during the optimization process. Two of the lens curvatures were used to maintain the lens power distribution between the two lens groups. The remaining (eight) lens curvatures, along with the curvature of the image surface, were used as variables during optimization.

Spherical aberration, coma, distortion and astigmatism were the aberrations controlled during optimization. It was found that a balance between third and fifth order spherical and focus would result in satisfactory on-axis performance. Coma and astigmatism were controlled to produce the best possible off-axis performance. Toward the end of the optimization process, a piano concave field flattening lens was added very close to the image plane to yield the final lens design (Figure 1a).

Figure 1. Original 6-element, 260-mm, f/3.0 lens design, generated in 1964.