Why It Still Feels Hot Even with 100% Light Blocking | How to Choose a Truly Cool Parasol by Understanding the Differences Between UV and Heat Shielding

How should you interpret parasol performance?
— The Science of Light Blocking, UV Protection, and Heat Shielding, and How to Identify a 'Serious' Parasol —
The moment you open a parasol under the intense summer sun, your body can suddenly feel more at ease. The glare softens, and you get the sensation that the heat is receding slightly. This change is not a coincidence; it is the result of the parasol controlling both "light" and "heat."

However, when you actually try to choose one, you are often met with labels like "100% light blocking," "99% or more UV protection," and "heat shielding effect," making it difficult to understand the differences. In this article, we will gently break down these differences and organize "how to look at specs that truly matter" in a simple way.

Why is a silver exterior and black interior logical?
First, let's start with a discussion of the structure, which is intuitively easy to understand.
A parasol with a silver (or white-based) exterior and a black interior is generally considered a logical configuration. The reason lies in the difference in how light and heat are handled.

Bright colors and silver have the property of easily reflecting the light and heat contained in sunlight. Therefore, the umbrella itself is less likely to get hot, and it is expected to have the effect of suppressing the intrusion of heat from above. On the other hand, black has the property of absorbing light, and by absorbing the reflection from the ground on the inside, it works to reduce the re-reflection onto the face and body.

In fact, it is explained that light-colored fabrics tend to reflect light and heat more easily, leading to higher heat shielding properties (Reference: [1]).
This structure can be considered a design that simultaneously implements measures in both upward and downward directions, by:
• Reflecting heat from above, and
• Absorbing reflections from below.


Think of parasol performance in terms of three indicatorsParasol performance is often explained from the following three perspectives:
• Light blocking rate: How much visible light is blocked
• UV protection rate: How much ultraviolet radiation is prevented
• Heat shielding rate: How much heat (mainly infrared radiation) is suppressed
While these may seem similar, they each target completely different physical phenomena. It is explained that light blocking is related to brightness, UV is related to the effect on the skin, and heat shielding is related to temperature, and each has an independent meaning (Reference: [1]).


Differences in measurement methods — The same "%" can have different meaningsOnce you understand this a little, your view of the labels will change significantly.
The light blocking rate is a test that measures how much light emitted from a light source passes through the fabric. The numerical value is calculated by aligning the light source, fabric, and sensor in a straight line and comparing the amount of transmitted light. Such tests are often conducted based on JIS L 1055 (Reference: [2]).
On the other hand, for the UV protection rate, a light source dedicated to ultraviolet rays is used to measure transmission for each wavelength with a spectrophotometer, and the evaluation is performed by considering the impact on the human body. It is explained that the method of JIS L 1925 is often used for this (Reference: [2]).

While light blocking is a test that looks at "brightness," UV is an evaluation based on the "impact on the skin."
And the heat shielding rate has a different nature. Here, simulated sunlight (such as a xenon arc lamp) is irradiated, and the temperature rise of a black reference plate placed underneath is measured. The temperature difference is compared with and without the fabric to evaluate how much heat intrusion was suppressed. It is explained that such tests are sometimes conducted based on methods such as JIS L 1951 or methods specific to each testing organization (Reference: [3]).

In other words,

Light blocking and UV measure light transmission, and
Heat shielding is an indicator that measures the "result" of temperature change.
This difference has significant meaning.


The heat shielding rate is closest to the actual feelingWhen the purpose of using a parasol is to "reduce heat," it is often said that the heat shielding rate is what most easily affects the actual feeling.
In verification cases, it is introduced that there are cases where there is a difference of about 5 to 8°C in perceived temperature depending on the presence or absence of heat shielding (Reference: [4]). However, since this depends on environmental conditions, it will not be the same in all situations.

Also, in some materials, it is explained that the heat shielding rate of general parasols is often around 35-40% (Reference: [5]). This is sometimes used as a guideline, but it is necessary to be aware that it changes depending on the product and measurement conditions.



Why the heat shielding rate is not often displayedHere, we get into a bit of a realistic discussion.
For many products, while the UV protection rate and light blocking rate are displayed as numerical values, the heat shielding is often limited to expressions like "heat shielding effect included" (Reference: [7]).
This is due to several reasons.

First, temperature is easily affected by environmental conditions such as wind and humidity, making the reproducibility of measurements difficult. Furthermore, it is explained that expressions like "it drops by X degrees" are treated with caution because the results change depending on the conditions, which could lead to misunderstandings (Reference: [6]).
Therefore, even though the heat shielding rate is an important indicator, the situation is that there are few unified displays.



How to identify specsGiven this background, the way you read the labels also changes a little.
For example, if only the number "100%" is emphasized, it is important to check what that refers to. Also, it is explained that expressions like "complete" or "strongest" should be treated with caution in a strict sense, as it is difficult to completely prevent the transmission of a small amount of light due to the product structure (Reference: [8]).
Reliable products tend to have explanations for multiple elements such as light blocking, UV, and heat shielding, and the performance content is often shown in a well-balanced manner.



SummaryA parasol is not just a piece of cloth, but a tool for controlling light and heat.
The light blocking rate handles brightness, the UV protection rate handles the impact on the skin, and the heat shielding rate handles temperature. Each has a role, and by combining them, "coolness" and "comfort" are created.
And structures like a silver exterior and black interior are an extension of that way of thinking.

When looking at numbers, rather than just whether they are high or low,
try to be slightly conscious of "what is being measured."
Just by doing that, choosing a parasol will become much more satisfying.
*This article is organized as a general trend by referring to various published explanatory articles and manufacturer information.



References (*Listed as reference information)[1] Amvel Co., Ltd. "What is the 'heat shielding rate,' the secret to parasol coolness?"
[2] Amvel Co., Ltd. "What is the light blocking rate of a parasol?"
[3] Amvel Co., Ltd. "Difference between heat shielding rate and heat shielding effect rate"
[4] New Trip "Explaining the meaning of UV protection rate"
[5] SPACECOOL Inc. "Basics of heat shielding rate"
[6] Precious.jp "Points to note about parasol heat shielding displays"
[7] Yomuno "How to choose a parasol"
[8] Amvel Co., Ltd. "Difference between complete light blocking and 100% light blocking rate"