How Can I Tell if My HMI Backlighting is Bright Enough?
“How bright should the product be… As bright as possible.” This is not a specification and can cost the company tens of thousands of dollars.
Many designers and engineers start the design for their HMI / UI, keypad, or display without knowing how bright the product needs to be. Often the prototypes look great and designers and management are happy, but the product has performance issues in the field. These poor performance issues are always costly and not easy to fix. Specifying the right backlight intensity at the beginning of the design can be an easy process and often keep the solution cost low while still meeting or exceeding the design criteria.
If the backlighting is too bright for the environment: text and icons will blur together and be difficult to read; hot spots are apparent; and the product can be a distraction and detract from the use and functionality. If the product is not bright enough: text and icons will not be easily visible or clear to understand; the product will be underwhelming and look under-powered and cheap.
Typical results from backlighting that is too bright or not bright enough:
- Dead front text and icons are too dim to read clearly
- Icons are not clearly lit to distinguish between on and off
- Hot spots are visible, and the unit looks cheap
- Display looks grainy and hard to read
- Higher total solution cost
At the start of the HMI / UI, keypad, or display design, the usage environment is known. When initial prototypes are reviewed in show rooms, labs, and design centers, this ambient lighting is usually not representative of the product usage environment. The human eye is adaptive and sees light relative to ambient light, so the backlight intensity must be appropriate for the usage environment.
Determining the specification for the backlight intensity can be started with some general guidelines, but ultimately will need to be selected by the designers. After using the guidelines to select a range of backlight intensity, a prototype or representative sample can be viewed in the usage environment and small changes can be made to fine tune and select the specification.
Low ambient light environments such as hospital rooms, outdoors during night, and automotive interiors will typically have backlight intensity between 40 and 100 cd/m2 (candelas per square meter). This will provide an easy to read product.
Mid-range ambient light environments such as homes and offices will typically have backlight intensity between 450 and 1,000 cd/m2. The typical location of the product will influence the final selection of backlight intensity. A kitchen and office tend to be brighter than a bedroom or laundry room.
High ambient light environments such as operating rooms, outdoors during daylight, and showrooms will typically have backlight intensity of 3,000 to 10,000 cd/m2.
Performance and Cost Drivers
The selection of the right backlight intensity can be an easy process. If proper selection of the backlight intensity is not done, it is likely the product will be too bright or not bright enough. Both scenarios will drive up costs in different ways. If the problems are not corrected, the product will underperform in the field and the OEM will have a quality perception issue.
Too bright is a unit cost problem. There will be un-necessary LEDs or a more expensive solution than required. The performance issue can be corrected by reducing the LED brightness, but this will still waste an average of $0.50 to $2.00 per unit. To correct the waste issue, a redesign is likely required.
Not bright enough is a larger problem than too bright. This is not simply a unit cost problem. Correcting a not bright enough product will take a redesign of the solution. A redesign of the PCBA, light panel or light guide, increased LEDs, and LED drivers are likely required. A redesign of the facia and housing are possibly required. This cost can be thousands or tens of thousands of dollars.
The backlight intensity should be specified early in the design process. It is an easy specification to define based on the known usage environment. If the backlight intensity is not specified correctly, the result can be higher cost per unit, poor performance, and a redesign.
Starting with the backlight intensity guidelines, prototypes or representative samples can be quickly tuned to achieve the desired lighting in the usage environment. These simple steps at the right time in the design process will yield a great looking part for an appropriate cost.