What is a Specification Sheet?
What is a Specification Sheet? It is a list of parameters or specifications that define the performance of the lens. It’s usually the start of any lens design.
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Two lens design stories
Imagine this:
You’ve got a big lens design project. Your design will be used in this new exciting product, and the optics is the key device in the product (Think: iPhone camera, self-driving car LiDAR system, cinematography lenses). There is a lot of investment in the project, and you’re responsible for the lens design.
There are requirements for the optical performance, and you got a specification sheet to base your design. But you made assumptions of your own, and went forward with a lens design without checking with any of your colleagues, or what the customer wanted.
You did minimal research for the product and how it is used, so you missed some key features in the design. The customer isn’t that familiar with all of the optical phenomena, so there was a small but crucial omission in the specification requirements.
The design ended up being out of spec by about 15%, and due to the time schedule of the project, there was no way to recover and fix the problem. All because there was no care taken to check the specifications in front of you. That’s got to suck.
Okay, let’s paint a different scenario.
You’ve got a big lens design project. Your design will be used in this new exciting product, and the optics is the key device in the product. There is a lot of investment in the project, and you’re responsible for the lens design.
There are requirements for the optical performance, and you got a specification sheet to base your design. The specifications are very clear, with clear performance goals. At the same time, you realize that some specifications are in contradiction with each other. So you point this out very early in the project, and make sure which performance is negotiable and which is non-negotiable. You double-down on the one or two specifications that were the key to its performance. This made for regular conversation with the project manager of the team, the project manager of the customer’s team, and very timely updates in the progress of the lens design. The project left no stone unturned, and becomes a success.
Due to the success of this project, more confidence was given to your company by the customer, and more confidence in you. From then on, you have regular meetings with new projects and exciting opportunities.
I think we all know which situation is better. Definitely, the second story is better.
And I should know, because the two stories that I just told you are both about me, and I don’t want you to make the same mistake that I made. On the flip side, I want every lens designer to be able to experience what I did, which is the adulation of a client due the immaculate work that you put in. This results in the complete trust of your skills from the client or your boss, and they come back for more and more projects and interesting work.
There are many common threads in the story, but the one I want to focus on (pun intended) today is the specification sheet.
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I’ve written about classical methods of lens design, underlying a lot of applicable theory, but this is a more hands-on approach to the tools of lens design compared to lens design itself.
Have you ever gotten a specification sheet? What is a specification sheet?
- Maybe a client you were working with gave it you a list of specifications for a lens design.
- Maybe you were given an RFQ (Request for Quote) that had optical specifications that had to be met?
- Maybe another division in your company gave you a specification sheet to design, for an overall project for your company.
- Maybe your boss made a list of optical parameters comparing a competitor’s product.
All of the above examples is that the specsheet is made by someone else, and you abide by it.
Not all specification sheets, from customers or otherwise, are complete. What if the customer is not aware of a crucial optical property, to no fault to them? Do you leave it out saying “not my problem”, or are you the professional that confirms the need of said specification?
For every lens design we do, we can add our own past experience into the specification, since we are professionals that bring the nuances of lens design. What if one crucial specification makes or breaks the lens design?
Using the specification sheet to guide our lens design
We can also use the specification sheet to guide us through lens design. Let me explain.
The specification sheet will give us the parameters we need to fulfill, and we will set up our lens design accordingly. A wide angle camera lens with a small aperture is going to have a different lens configuration than a narrow-angle camera lens with a large aperture, obviously.
However, even a similar lens can look completely different depending on what the needs are. Take our wide angle camera lens, for example.
The same type of lens with different requirements
A symmetric wide angle lens
This lens is good for:
- Wide angles
- Deep depth of field
- Low distortion
- High center resolution
- Compact lens size
This lens is NOT good for:
- Relative illumination (it has dark corners)
- Long back focal length (length of the last lens to the image plane or sensor)
- Small CRA or chief ray angle, the angle the light hits the image plane or sensor (image sensors require a minimum CRA)
A retrofocus wide angle lens
A retrofocus wide angle lens requires a longer backfocus, consequently may have some distortion, and a large front lens with large asymmetry.
This lens is good for:
- Wide angles
- Deep depth of field
- Long back focal length (length of the last lens to the image plane or sensor)
- Relative illumination
- Small CRA or chief ray angle
This lens is NOT good for:
- Compact lens size (the backfocus is large)
- Low distortion (the asymmetry in the system makes it difficult to correct distortion)
It all depends what the required performance of the lens is.
The lens design specification cheat sheet
Given the above stories and examples, how do we take the specification sheet to our advantage? How can we make the perfect lens design, by taking the specifications and getting the most out of the design?
I say that we make a specification sheet template of our own. I call it a Specification Cheat Sheet.
What does a “cheat sheet” mean? It means that we can use it to get an unfair advantage in the lens design. Granted, “unfair” is harsh word considering that we’re putting in the hard work to make exceptional lens designs! This specification “cheat sheet” we keep with us and use as a reference for any future project. Every time we get a lens design project, we reference it to get the best design possible in the least amount of time.
Making your very own specification cheat sheet has many advantages:
- We can check what the lens design needs, in order of importance
- We can find missing parameters of the lens design at a glance
- We can use the cheat sheet at the developing stages of a project to compare with past lens designs
Also, there are many use cases for a template of a specification cheat sheet.
- We can open the cheat sheet alongside the specification sheet from our client, compare the two and see what parameters are missing, and add it to the client specification, and contact them about the missing parameter. (Bonus points: if there is a parameter that is NOT on our specification sheet that is important to the project, we can add it and update our specification cheat sheet)
- Let’s say we’re at a client meeting, and when we are discussing the parameters of the lens design, we can whip out the specification sheet, all ready to check right there during the meeting, negotiate and explain the crucial parameters on the spot.
- We can open the cheat sheet alongside our lens design, and make adjustments to our lens design as a reference, as we are optimizing the performance of the lens.
- We can bring the specification cheat sheet to an in-company production meeting, where we can discuss negotiable and non-negotiable specifications in the system. (with other departments, for example)
What’s also great is that we are bound to learn new things as we finish a lens design. That means we can update the specification cheat sheet as time goes by, and it will be tailored to our very needs.
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What’s inside the lens design specification cheat sheet
Now we know the benefits of a specification cheat sheet. What are the items in a typical specification sheet?
I want to break the specification sheet down with two examples:
- Imaging lens specification sheet
- Illumination lens specification sheet
1. Imaging lens cheat sheet for imaging optics
I have more experience in smaller imaging optics such as mobile phone lenses and very small optics, but the same applies for large lens systems with many lens elements and expensive glass, as well as zoom systems. Imaging lenses have been developed for a century, and there is a lot of innovation but a lot of history as well, and a lot of what applied in the past also applies today.
What you can include in an imaging specification cheat sheet:
- Important optical parameters of the lens and optics
- The most basic of parameters for imaging such as the focal length, field of view (FOV), number of lens elements, total length of the lens, the F/# of the lens.
- Important system parameters for mechanics
- Total length of the lens, the back focal length of the lens, the largest possible lens element radius.
- Important performance parameters
- MTF, relative illumination, depth of field, the measurable parameters when the product is in prototype or manufacturing.
- Important environmental changes in the system
- Temperature, humidity, shock, etc.
- Other special parameters specific to the lens design
- Is it an inner-focus system zoom lens? Is it a fixed focal length lens? Is there an inner element to handle image stabilization? Are there aspheric surfaces? Exotic glass?
2. Illumination lens cheat sheet for imaging optics
The second cheat sheet is for illumination lenses, where a non-sequential raytrace is involved. I have a lot of experience with very complex lens structures with many degrees of freedom non-straightforward shapes. This genre of lens design is newer than classical imaging lenses, and with innovations in semiconductors we now have lighting systems that range from lasers and LEDs, with wavelengths of white light to UV and infrared.
What you can include in an illumination specification cheat sheet:
- Important optical parameters of the light source
- Is it an LD (laser diode), or an LED? Or less commonly, is it an incandescent light bulb, fluorescent lamp, the sun? There are some other variations within each one, such as VCSELs for LDs, White LEDs versus monochromatic LEDs, White LEDs with a blue chip and yellow phosphor, or RGB White LED, which all have different properties.
- How many light sources are there, or are at our disposal?
- (for LDs) Is it single mode? Or multi-mode?
- For LDs. Do we have to take into account for wave properties such as interference and/or diffraction?
- What is the dominant wavelength of the light source?
- The unit of measure of the system
- Photopic:
- Illuminance (Lux=Lm/m2)
- Intensity (Lm/sr)
- Luminance (Lux/sr)
- Radiometric:
- Irradiance (W/m2)
- Radiant intensity (W/sr)
- Radiane (W/m2/sr)
- Is there a requirement for the FOI? (FOI = field of Illumination, or the area of illumination, also sometimes referred to as the FOV – field of view)
- What is the requirement for the target illumination?
- Ex. 1: Flat top, uniform distribution across the FOI
- Ex. 2: The efficiency of the system is more important, so prioritize power over distribution.
- Ex. 3: The target intensity is a function of cosine(theta)
- Important system parameters for mechanics
- Is there a constraint on the diameter of the lens and the overall thickness of the lens? (make sure to double check for glass covers that add to the overall size, electronic devices in the vicinity of the light source that need to be avoided, etc.)
- Is there a requirement for the number of lenses to be used? (for example, 1 lens for cost/space reasons, or transmission reasons)
- Is there a requirement on the front lens element? Should there be a cover glass to avoid scratches, or a hard coating?
- Important performance parameters
- The target illumination above
- Is there a requirement for the light gathering ability of the lens such as F-number and/or n.a. (numerical aperture)?
- This is usually not required for illumination systems, but if the system includes a collimator or microlenses, these definitions are useful to get an idea of the performance of the lens or the optical system.
- How much transmission and/or efficiency is needed? Is an anti-reflective coating on the lenses a requirement?
- Make sure to check the environmental requirements, especially temperature, as some anti-reflective coatings cannot withstand large temperature changes.
- Important environmental changes in the system
- Temperature, humidity, shock, etc.
- Any requirements on dust-proof, waterproof, shock?
- Other special parameters specific to the lens design
- Is there a RayData file for the LED?
- Is there a intensity diagram for the LD?
Parameters for your client, and parameters for yourself
Some parameters are for your client, such as the performance of the lens, they need to know so that they know what to expect when they put your lens design in to their optical system.
However, you can also have parameters in the lens design that you only share with yourself (and the people in your team/company). For example, the methodology you used to optimize the performance, or in-house information that only your company knows about.
Useful resources
I have a few examples of useful resources that are sure to help you make the most of your specification cheat sheet.
Field Guide to Geometrical Optics by John E. Greivenkamp
This is one of my go-to books for not only lens design, but optical talk with colleagues, since it is a handy pocket-sized book that can be brought anywhere, and used as a reference. I mention this book a lot in my Ultimate Guide to Lens Design Using Spreadsheets, so you if you’ve been reading my guide you know it’s one of my favourites 😉
Field Guide to Lens Design: Julie Bentley; Craig Olson
There are steps to lens design in this book that uncovers the veil cast on lens design based on theory and software optimization, written simply for people like me who are practicing physicists (and engineers, scientists).
For example, although the book is light on the mathematical derivations, it has an outline of the general lens design process. There are sections devoted to tactical steps such as improvement of lens performance, tolerancing, stray light, and optical materials, all of which are more hands-on that pure theory. Great book, highly recommended.
Modern Lens Design, Warren J. Smith
This was the book I bought when I started my job as a lens designer, it is packed with information and a real dense read. I hear there is a third edition coming out, which is going to be revised from start to finish, updated to modern standards with new material like cell phone cameras and 360 degree lenses. By none other than Julie Bentley and Craig Olsen above.
Modern Optical Engineering, Warren J. Smith
The second half of this book was more useful for me in terms of specification of a lens design, as there are a slew of practical knowledge in the latter sections of the book from materials, system layout, lens design examples, and testing.
Optical System Design, Robert E. Fischer; Biljana Tadic-Galeb; Paul R. Yoder
Another book I bought just as I started my career as a lens designer. There are several tips and tricks that I use that came out of this book, and identifying the specification sheet is not excluded.
Field Guide to Illumination, Angelo V. Arecchi; Tahar Messadi; R. John Koshel
I frequently reference Field Guide to Illumination. As I’ve stated before, Field Guides are my go-to for quick references, as they are laid out so that information is easily accessible. This one is about illumination systems.
I frequently look up page 7 of this Field Guide, where it shows the four spatial quantities and the three spectral categories of illumination in a matrix format. Terms such as “illuminance”, “irradiance”,”intensity”, “luminance”, all have different meanings, and I usually do a double-check of the quantity that I need to refer to in a lens design.
Illumination Engineering: Design with Nonimaging Optics, R. John Koshel et. al.
This is a more in-depth book than the Field Guide, and it is authored by many leading figures in the industry today.
Two lens design specification cheat sheets for you!
I’ve went on long enough, I have something special for those who have read this far. I have prepared two specification cheat sheets that you can use for yourself! It has the very essentials of lens design for imaging and illumination. You can follow the cheat sheet to gain new perspective on lens design by thinking about the parameters that I present here. Also, if you’re not as experienced lens design, you can use these cheat sheets to check if you’ve missed anything in your lens design. If you’re just starting in illumination lens design, this cheat sheet will give you an idea of what you should look for in a lens design for illumination.
Of course, like I’ve stated previously, you can do whatever you like with these cheat sheets, you can add anything you want, as you gain knowledge and expertise in your field.
The beauty of your customized specification cheat sheet is that you get to update it as you go. And you can always leave out what you don’t need at the moment. At the same time, whenever you finish a project, you can add what you learned and put it in your cheat sheet, to make it your own!
I really want people to use some frameworks in your workflow to not only make your job as a lens designer easier, but to also make sure that we don’t make fatal mistakes that can delay a project or result in a project that is not fulfilled to its best ability. Most of all, I don’t want people who visit this website to make mistakes that can take a hit to your credibility, and make future work difficult for you. It happened to me, so I don’t want it to happen to you.
Download your very own specification cheat sheet, I have two ready for you, imaging lenses and illumination! (Just fill out the form below to let me know where to send it)
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One more thing…
If you liked this piece, please have a look at my Ultimate Guide to lens design: Using the classic spreadsheet method, a 24,000 word piece on how to do lens design without the use of expensive, proprietary software so you can get into the mechanics of lens design.
Alan says
Hi,
I just found your http://www.pencilofrays.com website on lens design a few days ago. I have played around with Winlens/Qioptic at an amateur level for quite a few years, but there is a lot that I still need to learn, and I am hoping that your tutorial materials will be a great help. So far they are looking promising. By the way, as a specific question, do you have the design parameters for the famous Zeiss 58mm f/2 Biotar, including glass types, surface definitions, and spacings? That was one of my favorite lenses, and it would be interesting to see what it looks like in simulation.