Digital Optical Trial Frames
Optical Engineering Solutions Ltd ("OES") are pioneers in the world of digital optical trial frames and Beta Solutions was privileged to partner with them to develop their first prototype. These digital frames overcome some significant limitations over their traditional "analogue" counter parts and propel trial frames into the 21st century.
Anyone who has had their eyes tested by an optometrist recently, will likely be familiar with "Trial Frames". Trial frames are an important tool for optometrists as they quickly enable them to try various combinations of lenses on the patient before settling on a final prescription. Among other things, trial frames will generally have the ability to:
- Adjust the lateral position of the Lens (pupillary distance).
- Generally measured to the nearest millimeter and indicated by a simple linear "ruler-like" display.
- Adjust the angular orientation of the lens.
- Generally measured to the nearest degree and indicated by a simple "protractor-like" display.
While existing analogue trial frames can be adequate, they do have some significant limitations. Namely:
- In the darkness of an optometrist examination room, it is often difficult for the optometrist to quickly read off the indicated measurements.
- If the optometrist happens to be reading the measurement at "anything other" than a right angle, it is possible they may slightly mis-read the measurement due to parallax error.
The founder of OES (himself an experienced optometrist) recognised these short comings, and thought there had to be a better way. OES then turned to Beta Solutions, in collaboration with other 3rd party developers, to design and develop a Trial Frame product with the following key capabilities:
- Sense the pupillary & angular distances to an accuracy of 0.5 mm and 0.5° respectively.
- Indicate the measurement to the user via an aesthetic illuminated display.
- Be as compact and light weight as possible.
Example of Analogue Optical Trial Frames
One of the first steps in the design process was to devise a sensing technology which would met all of the primary objectives. Many technologies were considered and assessed according to their advantages and disadvantages. For example, optical encoders have the capability to accurately sense linear and angular distances, however they are often bulky, heavy and expensive.
Capacitive sensing was the technology ultimately chosen for the following reasons:
(Very frequently the technology of choice for precision Digital Calipers)
- Light weight / compact
The PCB consisted of the following several key components:
The main PCBs accommodated all of the active components - such as the microprocessor, power supply circuitry, capacitive sensing and display screens. Very tight design constraints were placed on the main PCBs due to the compact nature of the product. Additionally, the PCBs had to seamlessly integrate with the equally tight constraints of the mechanical frames.
To reduce the size of the PCB (thereby increasing density), we implemented the following techniques:
- Multi layer PCB (4 layer)
- "Blind vias"* (in some cases)
- Small surface mount (SMT) components - such as 0603 resistors and capacitors
*Blind vias are a feature where the conductive hole in the PCB (connecting the copper layers together) only penetrates partly through the PCB, as opposed to going through the entire PCB (refer to image below). This then, frees up the opposing side of the PCB to accommodate more circuitry.
Capacitive Sensing Circuitry
A capacitor is formed when two overlapping conductive plates are separated by some (usually small) distance. When a voltage is applied across these plates, an electric charge becomes present, which then can be measured. Hence, the term capacitive sensing.
Simply put, if the voltage across the plates and separation properties remain constant, one can still change the amount of electric charge (capacitance) by varying the effective area of the over lapping plates.
The digital trial frames build significantly upon this concept, and use a host of smart techniques - including signal processing - to ultimately end up with the measured capacitance being translated to a linear or angular distance.
Power Supply Circuitry
As the device was required to be as light as possible - we opted to power the circuitry from a rechargeable Lithium-ion (Li-ion) battery. Li-ion batteries have the highest energy density of all rechargeable batteries.
While there were many possible display screen options, ultimately an OLED screen was selected. OLEDs have the following properties:
- Light weight
- Ability to illuminate the pixels
- High resolution and readability
- Low power consumption
... Compared to some other illuminated screen technologies
Microcontroller and Firmware
As usual the micro-controller is where the computing intelligence resides. The micro - under the instructions of the operating firmware - receives the incoming capacitive signals, performs real time signal processing and mathematical calculations, and displays the result onto the OLED display screens.
We worked very closely with specialist mechanical product designers, who oversaw the design and development of the Frames. Both parties were able to harness the power of modern CAD software to model the PCB and Frames in 3D. The 3D model could then simply be electronically communicated to the other party for inspection and validation.
After several optimisations, the 3D Frames model were ready to be turned into a physical prototype, which consisted of 3D printing certain aspects and CNC machining other important parts.
For the PCB, we were able to utilise our in house prototyping facilities. This resulted minimal tooling costs for OES and (equally important) a rapid turn around time for the prototype.
A prototype was successfully delivered to OES, which was capable of sensing the linear and angular measurements - to an accuracy of less that 0.5 degrees. Further, the entire electronics (complete with PCBs, components, display screens and battery) ended up weighing only 15.5 grams. OES are now in the process of pressing forward to production and the full commercialisation of this game changing product.
* All the content in this article, including imagery, has been approved by our client for publication on this website.