Motorized semi-automated prism bar technique for enhanced prismatic evaluation

Abstract

This article presents the development and functional design of a motorized semi-automated prism bar intended to enhance the accuracy and reliability of horizontal fusional vergence testing. Conventional prism bar techniques are prone to several limitations, including examiner-dependent variability, incorrect prism positioning, and discomfort for patients due to improper handling or inconsistent distance from the eye. These factors contribute to significant measurement errors. Additionally, even small deviations in prism placement or holding angle can lead to large discrepancies in prism diopter readings, ultimately affecting diagnosis and therapy outcomes. To address these challenges, a cost-effective, semi-automated prism bar system has been developed using a 24-V DC motor, lead screw, gear set, stroke rod, and limit switch mechanisms. The motorized device allows linear movement of the prism bar in 1.5-cm increments with the push of a button, ensuring precise alignment with the patient’s eye. A chin-and-headrest system maintains a constant prism-to-eye distance, minimizing human error and enhancing patient comfort during measurements. The system incorporates limit switches to prevent overextension and employs a control unit to manage DC power supply efficiently. This innovation meets the clinical needs for fixed prism positioning, thus improving accuracy and repeatability.

A prism bar is a valuable optical device with horizontally and vertically stacked prisms designed to test base-in, base-out, base-up, and base-down positions for step vergence.

Inaccurate results can occur if a prism bar is used in a position where it is not calibrated or if the demarcation line appears anywhere other than the center of the prism. Therefore, ensuring the accuracy of the prism bar is crucial when measuring squint for diagnostic purposes, pre-surgical evaluations for squint correction, and vision therapy.[1]

This device is also essential when precise fusional vergence measurements are required, as abnormalities in fusional vergence can lead to various asthenopic symptoms.[2] Hence, holding the prism bar in the correct position is of utmost importance. This can be particularly challenging for inexperienced examiners or non-cooperative patients, potentially leading to inaccuracies in orthoptic investigations.[3]

Automating prism bar techniques is essential to minimize errors and improve measurement accuracy. Additionally, maintaining the correct distance between the prism bar and the eye is critical, as the distance at which the prism is held affects its effectiveness. The farther the prism is from the eye, the lower its effective deviational power.[4] Any change in prism positioning alters the angle of ocular deviation measurement.[5] Prism-induced changes in visual perception can sometimes cause patient discomfort or confusion, impacting the reliability of the results.[6]

The amount of deviation neutralized by an ophthalmic prism can vary depending on how the prism is held. For instance, a 40Δ glass prism with its posterior face held in the frontal plane provides only 32Δ of effect.[7] Wang et al. demonstrated measurement errors caused by rotating both glass and plastic prisms away from their calibration positions, with larger errors recorded for glass prisms. They also recommended standardizing prisms used in strabismus measurements with the aid of a prism frame to eliminate positioning errors.[8]

Considering these factors, a motorized, semi-automated prism bar has been developed. It employs a linear actuator to precisely move the prism bar to the next prism with a simple button press. Additionally, a chin-and-head rest ensure that the correct distance is maintained between the prism and the patient.

This article presents the construction and development of the semi-automated prism bar, focusing exclusively on horizontal fusional vergences.

Methods [Video 1]

Construction

The semi-automated prism bar comprises five main components: a motor [Fig. 1a], limit switch buttons [Fig. 1b], a lead screw [Fig. 1c], a gear set [Fig. 1d], and a stroke rod [Fig. 1e]. The 24-V, 55-W DC motor (model FY01-150-325-24-4-6000) plays a crucial role in generating the linear movement of the prism bar.

Figure 1.

Main components of the motorized semi-automated prism bar (a) The 24-V, 55-W DC motor (b) Limit switch buttons to ensure that the stroke rod reaches extreme positions (c) The lead screw’s primary function is to convert the motor’s rotary motion into linear motion (d) The gear set serves as a connector between the motor and the lead (e) The stroke rod is connected directly to the lead (f) A clamp to hold the prism bar securely (g) Chin-and-forehead rest

Limit switches ensure that the stroke rod reaches its extreme positions at both ends. When triggered, the limit switch cuts off the motor’s current, stopping the movement of the stroke rod. The movement ceases when the patient stops pressing the limit switch.

The lead screw’s primary function is to convert the motor’s rotary motion into the linear motion of the stroke rod. It is housed within the cylinder, with its upper part connected to the stroke rod and its lower part linked to the gear set. The gear set acts as an intermediary between the motor and the lead screw, transforming the motor’s high-speed rotation into the high-torque, low-speed rotation required for precise linear motion.

The stroke rod is directly connected to the lead screw. When the lead screw rotates in the forward direction, the stroke rod moves upward; when it rotates in the reverse direction, the stroke rod moves downward.

Beyond the motor system, the linear actuator includes a clamp [Fig. 1f] to securely hold the prism bar and a chin-and-forehead rest [Fig. 1g] to provide stability and comfort during measurements. Additionally, the motorized semi-automated prism bar features a control unit responsible for converting AC to DC, ensuring efficient operation.

Assembling of the semi-automated prism bar

Step 1: Motor and gear set integration

• Attach the DC motor [Fig. 1a] to the base of the linear actuator housing.

• Connect the motor shaft to the gear set [Fig. 1d].

• Secure the gear set firmly with screws. Ensure proper alignment to avoid friction.

Step 2: Lead screw installation

• Insert the lead screw [Fig. 1c] vertically into the housing.

• The lower end of the lead screw should be connected to the gear output shaft.

• The upper end of the lead screw should remain free to move the stroke rod.

Step 3: Stroke rod attachment

• Mount the stroke rod [Fig. 1e] to the nut on the lead screw.

• Ensure that the stroke rod moves upward when the lead screw rotates forward and downward when reversed.

Step 4: Limit switch configuration

• Fix two limit switches [Fig. 1b], one at the top and one at the bottom of the stroke rod’s path.

• Connect the switches to the control unit to cut power when the stroke rod hits its endpoints.

• Test by manually triggering each switch to confirm current cutoff.

Step 5: Clamp and rest assembly

• Fix the prism bar clamp [Fig. 1f] securely to the upper end of the stroke rod.

• Ensure that it holds the prism bar firmly and aligns it with the patient’s line of sight.

• Install the chin-and-forehead rest [Fig. 1g] to the main frame structure in front of the clamp.

• Adjust height and position to comfortably align the patient’s eyes with the prism.

Step 6: Electrical wiring and control

• Connect the DC motor to the control unit for power conversion (AC to DC).

• Wire two push buttons (for upward and downward motion) to the control unit.

• Test the system:

  1. Pressing the “Up” button should move the prism bar upward.

  2. Pressing the “Down” button should lower the bar.

  3. Limit switches should cut the motor when the rod reaches the top/bottom.

Working of linear actuator

The DC motor moves the stroke rod upward when connected to a positive DC power supply and downward when connected to a reverse DC power supply. Two limit switch buttons, which control the upward and downward movement of the prism bar, are attached to the linear actuator [Fig. 2], allowing precise alignment of the prism bar with the center of the eye. The limit switches prevent overextension by cutting power when the stroke rod reaches its limit. This precise and controlled movement ensures reliable measurements during optometric examinations, enhancing both patient comfort and ease of use for examiners.

Figure 2.

Working of the linear actuator

Working mechanism [Fig. 3]

Figure 3.

The prism moves exactly to the next position without the demarcating line coming into view

• When the “Up” button is pressed, the motor runs forward, turning the lead screw and raising the stroke rod.

• When the “Down” button is pressed, the motor reverses, lowering the rod.

• Once the rod touches a limit switch, power is cut off to prevent overextension.

• The prism bar remains aligned centrally with the patient’s eyes for accurate and repeatable measurements.

Notes

• Use soft padding on the chin-and-forehead rest to ensure patient comfort.

• Ensure safety covers over moving parts to prevent injury.

• Regularly check for wear in the lead screw and gear components for long-term functionality.

• Label buttons clearly for ease of use.

This setup allows optometry professionals and innovators to create a low-cost, precise, and ergonomically effective solution for prism bar testing in clinical or educational environments.

Using the semi-automated prism bar

The semi-automated prism bar simplifies the process of measuring ocular deviations by allowing precise and consistent linear movements with minimal manual effort.

1. Powering on the device

   • Connect the device to an AC power source.

   • Switch on the control unit, which converts AC to DC and powers the 24-V DC motor.

   • Ensure the limit switches are functioning correctly by observing if the stroke rod stops at the extremes when moved.

2. Preparing the patient

   • Ask the patient to comfortably place their chin and forehead on the rest supports.

   • Adjust the chin rest height to align the patient’s eyes with the level of the prism bar.

   • Ensure that the patient is looking straight ahead and is seated comfortably to avoid head movements during testing.

3. Attaching the prism bar

   • Securely insert the appropriate prism bar into the clamp holder on the stroke rod.

   • Use the horizontal or vertical prism bar depending on the type of deviation being measured (horizontal for esotropia/exotropia, vertical for hypertropia/hypotropia).

4. Operating the prism bar

   • Use the “Up” and “Down” push buttons on the control panel to move the prism bar vertically until it aligns precisely with the patient’s visual axis.

   • The motor will rotate the lead screw, causing the stroke rod to move the prism bar linearly.

   • Once the prism bar is in position, release the button to stop movement.

   Note: If the stroke rod reaches its upper or lower limit, the limit switch will automatically stop the motor, ensuring safety and precision.

5. Performing the test

   • Conduct the prism cover test, alternating cover-uncover movements while increasing prism strength gradually.

   • Observe the eye movements to determine the neutralizing prism power that eliminates deviation.

   • Fine adjustments can be made using the motorized movement to maintain central alignment with the pupil.

6. Post-examination

   • Gently press the opposite direction button to retract the prism bar to its resting position.

   • Remove the prism bar from the clamp.

   • Clean the chin-and-forehead rest for hygiene before the next use.

   • Switch off the control unit and unplug the device if no longer in use.

Benefits of using the semi-automated prism bar

• Hands-free precision movement, thus reducing examiner fatigue.

• Consistent and repeatable alignment, thus improving the accuracy of measurements.

• Enhanced patient comfort through smooth motor operation and stable head support.

• Efficient workflow, especially during multiple or repeat tests.

Discussion

The motorized semi-automated prism bar presented in this study effectively addresses several practical challenges associated with conventional prism bar testing. Its non-contact design and linear actuator mechanism allow for precise, repeatable alignment of the prism bar, reducing examiner variability and improving measurement accuracy. This is particularly beneficial for orthoptic evaluations, where consistent prism positioning is critical for obtaining reliable results.

However, the current version of the device has certain limitations. First, it is primarily suited for cooperative patients who can maintain proper head positioning using the chin-and-forehead rest. This limits its applicability in pediatric populations, uncooperative individuals, and patients with neuromuscular conditions or postural instability.

Another constraint is the need for manual operation of the push buttons. While this is a step forward from full manual handling, further automation could enhance hands-free operation. The system also requires a stable power source and trained personnel to operate and maintain the device.

Looking forward, several future enhancements can be explored to expand the device’s utility:

• Adjustable or retractable chin-and-forehead rests could accommodate a wider range of patients, including children and those with special needs.

• Integration of self-assessment or remote control functionality could make the system suitable for teleoptometry and home-based vision therapy.

• Automated prism strength detection and digital recording of measurements would further reduce human error and improve workflow documentation.

Despite these limitations, the device offers significant advantages in terms of cost-effectiveness, user-friendliness, and clinical accuracy. It holds great potential as a valuable tool for both teaching institutions and clinical practices seeking to modernize their orthoptic evaluation protocols.

Conclusion

The motorized semi-automated prism bar addresses the practical and clinical challenges associated with traditional prism bars. By ensuring accurate alignment, consistent prism-to-eye distance, and examiner-independent operation, it provides a reliable tool for measuring horizontal fusional vergence.

Conflicts of interest:

There are no conflicts of interest.

Funding Statement

Nil.