When building a machine vision system, the developers must consider the application, resolution, illumination, depth of field, field of view, processing speed, and -- always -- cost. Often, when non-optical engineers are asked to design machine vision systems, the result is either systems that fail to meet the user's needs or systems that are more expensive than they need to be, because the designer over-specified the components. Part of the problem is that designers don't understand that optical elements are made differently (and include different economics and lead times) than most mechanical or electronic components. In this article, we discuss the economics of off-the-shelf and custom optics for machine vision systems.

Why one lens can cost as much as 50

The vast majority of lenses are produced the same way today as they were made during World War II. Deterministic grinding and polishing machines can be used to manufacture lenses one at a time. These machines have their own associated expenses. This method is usually used for low volume manufacturing. Tooling is more of a consideration with deterministic polishing. This is still fairly new technology and not as prevalent as the conventional pitch polishing. Many lens blanks are blocked onto one tool and pitch is used to grind and polish the entire batch. Several tools can grind or polish at the same time on a single machine.

Making a single lens takes just as long as making several hundred lenses. And because, for common glasses, material is a small portion of the cost, making one lens costs about the same as making 50.

Once the lenses are polished, they must be tested. This usually involves test plates of specific radii. The optical shop will have some test plates, but if they need to make a new test plate, the costs of manufacturing the lens will increase. Special tooling may also be necessary for custom lenses, further increasing cost.

Customers who do not understand that lenses are made in batches sometimes ask to receive just the first few lenses that are made, assuming that this will save time. It saves maybe a day (due to testing time), but this is negligible compared to the lead time for the entire process. Manufacturing a simple lens can take on the order of one to three months. If test plates must be manufactured as well, then expect to add another month or two.

If you are in a hurry, off-the-shelf (i.e., "stock") optics are your best bet. Off-the-shelf optics are (by definition) available more quickly than custom-made lenses. Off-the-shelf optics are made in quantity, are in continuing production, and are designed into standard matrices that provide customers with a wide variety of sizes and focal lengths from which to choose. They will be on your optical supplier's shelf when you call. Because many different customers use the same lens, off-the-shelf optics allow an economy of scale, even when one customer needs only a few lenses.

Off-the-shelf video lenses are common. Edmund carries a wide selection of lenses designed specifically for machine vision applications. Usually a little bit of flexibility on one or more parameters allows for an easy selection of an off-the-shelf lens.

Prototypes

Prototypes typically need to be built quickly, so these are almost certainly better suited to off-the-shelf optics. As explained above, in prototype quantities, custom lenses are astronomically expensive and require long lead times. Also, when (or, if you're an optimist, if) the prototype shows that the design must be changed, you won't have to pay all the expenses of having another custom lens designed and made.

The easiest way to incorporate off-the-shelf elements into a system is to use them during the optical design, before starting the mechanical design. Most optical design software packages have off-the-shelf lenses preloaded into them. Typical design software gives a starting point with custom lenses when one optimizes all surfaces. Then one can force the software to replace the custom lenses with the closest off-the-shelf matches and allow air spaces to compensate.

Production quantities

For systems in production, off-the-shelf lenses may also make sense, but this depends on the application and the number of production units. As a general rule of thumb, custom lenses make economic sense only when one needs thousands of lenses. There are two major exceptions to this rule: when stock optics cannot meet the requirements at all; and when one custom element can replace a number of stock elements, resulting in a less expensive system.

Lead time is an obvious consideration for prototypes, but also needs to be understood for production. Sometimes a customer using a custom lens suddenly has a dramatic increase in business and needs to have twice as many lenses as they forecast. If this sudden increase in demand cannot be filled, it could shut down their assembly line. Although some manufacturers try to keep a safety stock of lenses, such stocks are more difficult to maintain for custom lenses than for off-the-shelf lenses.

Volume makes the difference

Volume is the key to deciding when to use off-the-shelf or custom optics. Low volume will always favor off-the-shelf elements, but as volume goes up the advantages diminish and other factors take over.

When a low volume of between 100 and 1000 pieces is needed, economy of scale still rules: stock lenses are cheaper. At this volume, off-the-shelf optics can save the customer from having to commit to a supply for a year or more. Stock lenses offer the considerable advantage of allowing the customer to buy on demand and have stock available in case volume increases. If you really need a custom solution, however, it can be done at a reasonable cost at this volume.

For moderate volumes of 1000 to 100,000 pieces, both custom and off-the-shelf elements are viable options. Lenses are generally not stocked in these volumes unless a need for them is forecast for a specific customer. Increases in volume are still easier to accommodate with an off-the-shelf option (because there is less risk in overstocking a stock lens than in overstocking a custom lens). The savings that might occur by using custom lenses start to be important at these volumes.

For high volumes above 100,000, custom lenses are almost always used. If elements can be eliminated, custom is almost exclusively used. This volume provides the customer with economies of scale for the custom lenses. The cost per piece to manufacture 200,000 pieces is not significantly less than the cost per piece of 100,000 pieces.

Stock is not always best

When ramping up to production, most product designs alter, but be aware that changing a finished optical design can be costly. If you change the lens, then inevitably the mounting must also be changed to accommodate any difference in focus. Even lenses with identical focal lengths can mount differently because a change in radius alters where the lens is mounted. Also, the optical design must be redone.

Engineering for these kinds of changes will have associated costs that may outweigh the savings of an off-the-shelf lens. If FDA or similar approvals are required, the validation involved in a design change also can lead to severe costs. In this case, sticking with an optimized custom would be more efficient than moving from one off-the-shelf lens to another.

Another consideration is the effect on tolerances. If one uses more elements to correct aberrations without using custom optics, then the stack-up of tolerances can decrease performance. Also, some designs require a specific tolerance for some element, which may not be standard to off-the-shelf optics. Sometimes a very specific focal length is required, or a specific lens form, such as a meniscus lens to correct aberrations -- these may not be available off the shelf. Special coatings are a popular reason for a custom lens. Sometimes designs require very low reflectance at a specific wavelength or an antireflection coating in the UV or near IR wavelengths. Sometimes there is no off-the-shelf solution and a custom lens is unavoidable.

Sometimes, however, a stock lens can be customized by "edging down" the lens (reducing the diameter) or custom coating. Edging down can be done quickly and often inexpensively. This is useful for mounting in an existing housing or accommodating space limitations. Custom coating a batch of lenses can usually be accomplished relatively quickly.