Depth finders known as flashers have been helping sport fishermen “see” underwater to find fish, grass, brush, and drop-offs for more than 20 years. Flashers use sonar technology, sending high-frequency sound waves into the water and measuring the return time and strength of the reflected waves to provide anglers with instantaneous detail about bottom features and likely fish locations. The information is displayed by a spinning disk that presents a bar of light moving around an analog readout dial. This visual presentation of digital depth data flashes as the disk rotates, similar to a conventional sonar screen—hence, the term “flasher” (see Fig. 1).

Mounted on the dashboard or bow of the boat, most dept finders use liquid-crystal displays or glow lamps for the depth readout. Flashers incorporating glow lamps have limitations stemming from the neon bulbs used as the light source. Neon bulbs require high-voltage circuits and are heavy, loading the drive motor of the spinning disk. They generally lack the durability to withstand rough waters, and, when the bulbs break or burn out, the user has to send the instrument back to the manufacturer, resulting in significant downtime and repair costs.

Light-emitting diodes (LEDs) have many advantages over glow-lamp technology, including greater durability and considerably longer lifetimes. Because LEDs are surface-mounted devices, they can be housed in significantly stronger, more compact packages. Finally, unlike neon bulbs, they do not load down the drive motor or require the support electronics for high-voltage circuits. However, due to inherent variations in brightness, structure, light distribution, and alignment, they have not been widely adopted as light sources in flasher devices.

Techsonic Industries (Eufaula, AL) has recently developed an LED-based depth finder that offers an improvement over glow-lamp technology and solves problems typically associated with the LEDs. Using holographic Light-Shaping Diffusers (LSDs) from Physical Optics Corp. (Torrance, CA), product engineers have been able to achieve uniform light distribution and very high diffusion rates with minimal loss, resulting in a high-resolution display that is brighter and easier to read than previous LED-based designs.

Display System Design
Engineers at Techsonic used an elliptical diffuser pattern by Physical Optics that gave them the required illumination, minimized the size of the LSD, and reduced costs. The 2.5 x 1.0-mm Light-Shaping Diffuser incorporated in the new Humminbird Jimmy Houston Pro Flasher is mounted on top of two LEDs located behind a rectangular viewing window in the spinning disk, generating the bar of light that indicates depth (see Fig. 2).

The output transmitted by the diffuser is not only shaped to match the target area but homogenizes the structure variations and high-intensity regions, or hot spots, that have been problems with these sources in the past. This results in an even illumination that makes the flasher display easier to read. The diffusers are antireflective, producing extremely low backscatter and contributing to improved transmission efficiency and image contrast.

Liqht-Shaping Diffuser
Because the LSD is a surface-relief hologram, the diffusion characteristics are based on refraction rather than diffraction. The devices are random, nonperiodic structures that shape the beam by precisely controlling the energy distributing along the horizontal and vertical axes. This allows the designer to match the LED light sources with the area requiring illumination.

More than 90% of the light passing through the diffuser is transmitted to the flasher’s viewing screen in a sculpted beam profile. The elliptical transmissive LSD used in the flasher maintains a 30° side-to-side distribution along the annular projection of the display while simultaneously achieving a high diffusion of the source output along the major axis of the array, eliminating hot spots.

A proprietary surface- relief holographic replication process allows mass production of the LSDs by a variety of methods similar to those used to produce security holograms. The diffusers can be embossed into a deformable material such as acrylic, polycarbonate, or a variety of other plastic substrates.

Bill Dusinberre

BILL DUSINBERRE is senior mechanical engineer at Techsonic Industries, 5 Humminbird Lane, Eufaula, AL 36027.