The SAE Heavy Duty Lighting Committee decided that a standard was needed for high-mounted turn signal lamps for the rear of large vehicles. Since most vehicles use the same or very similar lamp(s) for stop lamps and turn signal lamps it was decided to use the high-mounted stop lamp as the basis for the high-mounted turn signal lamp. Therefore high-mounted turn signal lamps were included in SAE J1432.
The photometric requirements and physical tests are the same for both lamps. Provision has been made to allow the use of yellow high mounted turn signal lamps when the required turn signal lamps are yellow. It should be noted, however, that red clearance lamps are required on the rear of the vehicle which precludes combining these two lamps in that scenario. The HMTSL can be installed adjacent to the red clearance lamp if the clearance lamp(s) are not installed at the lower rear sill of the vehicle.
The HDLC believes the photometric values for these lamps need to be higher than for the required CHMSL on automobiles due to the fact that they are mounted higher and the direction of viewing these lamps could be more toward the sun than a regular CHMSL. A review of a study conducted by the University of Michigan Transportation Research Institute (UMTRI) Authored by Paul L. Olson entitled Evaluation of a New High-Mounted Stop Lamp (UMTRI 870065) indicates that in fact there is a need for higher candela requirements when daylight observation is taken into account.
The study was conducted using two age groups of responders 25 to 45 years old and 45 to 80 years old. The test determined the response time for tungsten type lamps and LED lamps when viewed near and far and in darkness and ambient illumination. The results indicate that a tungsten lamp when viewed from afar had 44 misses for the younger group and 16 for the older group. In darkness there were no misses in these groups from afar. LED lamps were better but still had 8 misses in the younger group and 2 in the older group in daytime. If the aspect ratio was changed to a thin longer type LED lamp the results were 12 misses for the younger group and 4 for the older group. The results indicate a signal is more likely to be missed in the daytime than at night.
A comparison of response times indicated respondents viewing the tungsten lamp showed an increase in response time of 40% in daytime verses the response time at night. The LED lamp showed an increase of 29% in the same comparison. Both the tungsten lamp and the LED lamp had an intensity of 42.3 cd at H-V. The LED lamp was 20% brighter but was equalized by placing an 80% transmission neutral filter over the LED unit to reduce its output from 51 cd to 42.3 cd.
Based on this study, the photometric requirements for HMSL’s and HMTSL’s used on large vehicles have been increased by 60% from the baseline established by FMVSS-108 Figure 10 for required CHMSL’s used on smaller vehicles.
Another change is the ratio between the red and yellow values in Table 2. The traditional method of using 2 ½ times the values of a red lamp to obtain the yellow values are not appropriate for yellow LED lamps. The 2 ½ times ratio was based on the photometric transmission factor of the red and yellow materials used in the plastic lenses. Saturated colors all appear brighter than they should relative to a photometrically equivalent white, and among colors, red is perceived as brighter than yellow when they are photometrically equivalent. This means that a yellow lamp that is about 1.6 times the photometric value of a red lamp should look about equal to the human eye.
The references to multiple lamp arrangements was removed from the standard because heavy duty vehicles, such as trailers, use industry standard lamps that are replaceable from several manufacturers and with several varieties of lamps, it is possible to replace one lamp of a multiple lamp arrangement with a lamp that would cause the arrangement to be in non-compliance with this standard, therefore each supplemental HMSL and/or HMTSL installed on the vehicle must comply with this standard.