Five Ways a Bad Exit Window Kills a Great Scanner
A checklist for the one critical optical component that most OEM designs get wrong.
By Kevin Chen, Design Engineer
You’ve designed a great scanner that works well on your workbench. Every test scan completes without error. But when you move to production, suddenly the decode performance suffers.
Is it poor illumination? An incorrectly configured scan engine? Is there some design flaw you missed?
In my experience, it’s usually due to a problem with your exit window.
What appeared clear to your eye may be causing internal reflections for the scanning engine. Now hundreds or thousands of units are experiencing decode performance degradation in the field.
A poor exit window is where good scanners go to die.
Why Does the Exit Window Matter So Much?
The exit window is where all the illumination leaves the scanning engine and where the engine expects to receive a clear image in return. It often becomes a single point of failure due to five key points I’m going to outline below.
When you’re testing a new design, you may not have the engine inside an appropriate enclosure to approximate how your design will respond in the field. The moment you enclose the scanning engine, the exit window becomes part of the optical system, just like the lens or sensor. It’s integral to the entire scanning process.
If you’re like most OEMs, you probably have mechanical, electrical, and software engineers on your team. The missing team member most often is an optical engineer who can provide guidance about one of the most critical parts of your build. That’s what I do for Dolphin and our customers.
Optical engineers use a formula called Snell’s Law that describes mathematically how light bends or refracts as it goes through various substances, including glass and plastics. This will matter when you’re choosing the material for your exit window. Don’t worry, I’m not making this a test. For your scanning engine, there are plenty of places to look up what you need to know for your design.
But as a design engineer, I know that the “ideal exit window” doesn’t really exist. It would need to be “zero by zero” or zero thickness by zero distance from the scanning engine plane. Since that’s not actually achievable, every design represents a set of tradeoffs against that ideal.
Five Ways a Bad Exit Window Kills a Great Scanner
Here are the top five ways you can improperly configure an exit window that can negatively impact your scanner design and what you can do to avoid each one.
1. Selecting the Wrong Exit Window Material
It may seem simple, but an optical scanning engine requires an optical-grade exit window. Plexiglass won’t work. While any clear material might look good to your eye, non-optical-grade materials can create distortions (like rainbow or wavefront patterns) and dim the engine’s illumination band.
The Fix: Optical-grade acrylic that has been matched to the engine’s illumination requirements or optical-grade glass for harsh, abrasive, or corrosive environments.
2. Too Thick or Too Far
Installing a window that is too thick or too far from the engine can cause different types of challenges for scanning. Thickness may distort your imaging, while distance can open a gap where stray light ricochets back into the sensor.
The Fix: Build an exit window as thin as your design safely allows, mounted as close to the engine face as possible.
3. Tilting Without Recalculating
Tilting the exit window or tilting the scanning engine can cause an optical problem, rewriting the light path.
The Fix: Keep your angle perpendicular when you can; when a packaging constraint forces a tilt, you should contact an optical engineer to guide you on your design or refer to the Zebra Integration Guide. We are here to help you.
4. A Bright Interior or an Uncoated Window
A reflective interior lets light bounce into the sensor; an uncoated window may act as a low-grade mirror. Either of these conditions can cause too much light to confuse the sensor.
The Fix: Make sure to use a matte-black interior for the chamber and an anti-reflective coating on the inside of the window. You should also use an anti-smudge coating outside where fingerprints can impact the window.
5. Picking the Window Color for Looks, Not the Sensor
As a designer, you may want to specify a colored window for aesthetics, but this can starve the illumination band.
The Fix: Choose a color that reads as intended to the human eye while preserving transmission for the sensor; test it under ambient light.
One step that I recommend before committing to final die-cutting and tooling for your devices is to craft a mock-up that allows you to test the exit window in as close to field environments as possible.
You can 3D print your design, paint the interior flat black, and insert the optical-grade exit window material. I have even crafted mock-ups using cardboard to simulate the body of the device.
During these tests, you want to run a camera check looking for mistakes. Many engines can stream a live image to a PC instead of decoding, so you can see problems that show up as bright spots (stray reflection) or distortion (wrong material). This type of pre-tooling and testing saves a lot of time and resources later on.
You can also contact us to run through the equations and select just the right materials for your end product.
Working with Dolphin Data Capture
We are recognized as one of the largest OEM barcode scan engine suppliers in the world for clients ranging from Fortune 500 enterprises and top-tier medical device OEMs to agile small and mid-sized businesses.
We integrate the world’s leading barcode technology into medical devices for top OEM medical companies — where accuracy isn’t optional and people’s health depends on it.
If we can solve problems for them, we know we can solve problems for you.
Contact the Dolphin team at 516-405-3990 or info@dolphindc.com and have your questions ready!