Analysis and countermeasures of fogging of automobile lamps

1. Background of Anti-fog for Car Lights

Anti-fog of car lights is an old topic, and it is a major quality problem that troubles lamp manufacturers, vehicle manufacturers and end customers. With the widespread application of LED light sources and the booming development of interactive control systems, car lights have become more electronic, digital and integrated, and their appearance has become thinner and more complex, with higher requirements for anti-fog, which has become the main problem facing car light design.

The mechanism of fog formation, leaving aside the technical terms, can be simply described as follows: the ability of the same volume of air to hold water vapor varies with the ambient temperature. The higher the temperature, the more water vapor it can hold. When the ambient temperature drops suddenly, the water vapor that cannot be contained by the air will condense into liquid water.

When the headlights are on, the temperature and pressure differences formed by the different speeds of temperature rise in different parts of the headlights cause the gas in the lamp cavity to flow and exchange with the outside world. When the outside humidity is high, the humidity of the entire lamp cavity increases. After the high-temperature and high-humidity gas enters the low-temperature area of ​​the lamp, the ambient temperature drops, the water vapor condenses, and forms fogging under the action of condensation nuclei.

2. The mechanism of fog formation in lamps

By analyzing the factors affecting the generation of fog in lamps, it can be briefly summarized that the fogging of lamps is mainly related to the temperature field, flow field and humidity field. The relationship between the three and the influencing factors are shown in the figure:

When the product shape and the materials of each component are fixed:

The temperature distribution inside the lamp is related to the power of the light source such as the bulb and LED and the external ambient temperature.

The flow field is divided into internal flow and external exchange. The internal flow is caused by natural convection caused by uneven temperature distribution inside the headlight and gaps between components. The ventilation structure in external exchange is mainly determined by factors such as the number, size and arrangement of ventilation holes and the type of ventilation components.

The humidity field is mainly related to the air humidity inside the lamp cavity and the air humidity outside. The internal air flow causes the humidity distribution to change.

3. Hazards of fog

Poor appearance, customer complaints, resulting in returns and exchanges, and increased quality costs

Affects the optical performance of the lamp. Water vapor condenses on the surface of the photometric mirror, causing refraction and deviation of light.

Affects the life of electronic components. As the number of electronic components in car lights increases and becomes more complex, water vapor condenses on the electronic components, causing them to fail over time.

4. New challenges facing anti-fog

1. Widespread use of LED light sources

The heating mechanism of LED light source is fundamentally different from that of halogen bulb. The light efficiency of halogen bulb is about 5%. 85% of the heat is dissipated in the form of infrared radiation, which is conducive to the even distribution of heat in the lamp.

The luminous efficiency of LED is about 20%. LED is a cold light source. About 75% of the heat is conducted to the substrate at the soldering point by heat conduction and then dissipated. The heat is concentrated, which is not conducive to the distribution of heat in the lamp.

2. The shape of lamps is becoming flatter and thinner, and the space is getting smaller and smaller

The shape of lamps is becoming more and more streamlined and flat, and the decoration parts such as decorative rings and decorative strips are becoming more and more refined. The structure of lamps is complex and the space is compact, which is not conducive to the flow of air in the lamp cavity.

3. OEMs’ requirements become higher

The OEMs have raised the test standards for fogging. The time it takes for fog to dissipate has been increased from the previous standard of allowing it to exist and being judged as qualified if it dissipates in half an hour to an hour, to now not allowing fog to appear. The requirements have been greatly increased.

5. 6 ways to solve the fog of lamps

From the conditions for the formation of lamp fog, it can be seen that lamp fogging is mainly related to the humidity, temperature distribution and condensation nuclei inside the lamp. Therefore, to solve the problem of lamp fogging, it is necessary to start from these three aspects and limit one or more of the conditions to effectively solve the problem of lamp fogging.

1. Reduce the humidity inside the lamp

For lamps with good sealing performance, reducing the humidity inside the lamp can be considered from three aspects: lamp material, selection of ventilation components and installation of dehumidification devices inside the lamp.

2. Water absorption rate of headlight materials

Try not to use materials with high water absorption, such as: headlight lens bracket, bulb holder and other parts try to avoid using PA series materials.

3. Selection of ventilation components

Breathable membrane

Breathing cap

Function: Isolate liquid water and dust, allow gas (including water vapor) to pass through, the breathable membrane allows a larger breathable area, in the form of a paste; the vent cap allows a smaller breathable area, in the form of a set.

4. Install a dehumidification device inside the lamp

Installing a dehumidification device inside the lamp, such as sticking a desiccant, can effectively reduce the humidity inside the lamp, thereby reducing the risk of fogging. However, this method also has disadvantages. The physical desiccant used will generally tend to saturate as the amount of moisture absorbed increases. After saturation, it will release the moisture it absorbs into the lamp when heated, aggravating the fogging phenomenon.

5. Balance the temperature field and flow field inside the lamp

The temperature field and flow field inside the lamp play a major role in fog. The structural design of the lamp will affect the temperature field and flow field inside the lamp. The following measures can generally be taken in the design to prevent it:

The utility model avoids the formation of a narrow structure between the lamp bezel and the photometric mirror area, especially the area below the lamp, where the narrow area is not conducive to the circulation of gas in the lamp, while meeting the heat resistance requirements of the lamp.

Try to avoid decorating large areas that are not directly illuminated by light sources; or make patterns on the inner side of the lens to reduce the impact of fog on the appearance.

A certain gap should be left between the upper and lower parts of the lamp to facilitate the flow of gas; it is usually necessary to avoid designing a narrow gap between the front (frame and mask) and the back (frame and lamp housing) of the headlight bezel, as narrow gaps will hinder the flow of gas.

CAE engineers should intervene early in the headlight design stage and use finite element analysis software to analyze and predict the fogging area and risk level of the headlights. By analyzing the temperature field and flow field inside the headlights, the design can be optimized to avoid low temperatures in the low-temperature area of ​​the light distribution mirror and gas flow dead zones, effectively preventing or solving the problem of lamp fogging.

The cooling fan is mainly used to dissipate the heat of the LED, which greatly improves the air flow and heat distribution inside the lamp, thereby bringing additional benefits such as fog improvement.

6. Reduce condensation nuclei

The condensation nuclei are usually reduced by spraying an anti-fog coating on the inner surface of the lampshade.

Mechanism of action: Make the water droplets spread on the surface of the substrate (the surface contact angle becomes smaller), forming a very thin water film, preventing them from condensing into water droplets on the surface of the substrate, reducing the scattering of light on the surface of the water droplets, and increasing the light transmittance.

Water drop contact angle on substrate surface before and after anti-fog coating

Since the anti-fog coating is sprayed on a transparent material, if there are quality defects, it cannot be treated by grinding and polishing and then repeated spraying. Relatively speaking, the product scrap rate is high. At the same time, the spraying environment requirements of the anti-fog coating are high, which will increase the cost of the lamps compared to other solutions.