Artificial Lighting for Orchids: Part Six – The Modern Way to Measure Artificial Lighting!

There is a lot of confusion about how to measure light intensity, especially among orchid enthusiasts, who often use outdated and unsuitable scales. In this column, we will attempt to demystify modern measurement concepts and show how they are much more useful and revealing of plants’ actual needs.

Maximum illumination at midday has long served as a reference point

In their natural habitat, orchids receive highly variable amounts of light throughout the day. When it became necessary to quantify the light intensity required by plants more rigorously, it seemed logical to use maximum illumination (around noon) as a reference. This is actually the maximum illumination that the plant can tolerate in its natural environment, a value mentioned in reference books. In practice, the intensities mentioned far exceed the average needs of plants.

Although the daily maximum provides a useful basis for comparing lighting requirements (low, medium, or high), it is not appropriate to use these extreme intensities to create a suitable growing environment. If such intense lighting were provided continuously throughout the day, the plants would become saturated with light and their foliage could even be damaged in just a few days.

Daily Light Integral (DLI) is a much better representation of actual lighting requirements.

With artificial lighting, light intensity remains constant from morning to night. To determine the equivalent continuous illumination, analysts calculated the daily light integral (DLI) under the bell curve, then distributed this energy evenly over a twelve-hour period.

Other natural factors also reduce the need for artificial lighting.

Cloud cover and natural obstacles (such as large trees that cast shadows) reduce the intensity of light available to plants in natural environments. However, these obstacles to light transmission are absent in domestic lighting. There are no clouds in our homes, and modern light fixtures provide more uniform lighting with little shadow, especially if they are installed correctly, as we recommended in our previous articles.

Another geometric factor is crucial for calibrating the intensity of artificial lighting. This is the “cosine” factor. As the sun crosses the sky from east to west, foliage is illuminated at several different angles throughout the day, with some projections being very effective and others much less so. This is because leaves, which are relatively flat and fixed surfaces, capture maximum light when the rays strike their surface perpendicularly.

On the other hand, only a small portion of the incident energy can be captured when the rays strike the leaf surface parallel to it. Over the course of a full day, there will therefore be a loss of efficiency in energy transfer of around 50%, regardless of the specific orientation of each leaf. This result is obtained by including the cosine of the angle of incidence of the light rays relative to the normal of the leaf in the calculation of energy transfer.

For continuous lighting, 15% to 25% of maximum intensity should be sufficient.

The light intensity recommendations found in most reference books on orchids are therefore 4 to 6 times too high for the actual needs of orchids “under constant lighting.” These intensities are more indicative of the maximum lighting that the plant can tolerate in its natural environment (at midday). However, in our domestic settings, the level of light required for orchid growth is much lower, as it is more constant, better distributed, and unobstructed.

It is not uncommon to see professional facilities (e.g., commercial greenhouses) that provide only 10% of the maximum light listed in traditional references. This low level of light is often sufficient to allow young plants to grow steadily without undue stress. As they mature, orchids will tolerate more intense light and can use it to promote flowering, hence the recommendation of 15 to 25% for healthy adult plants. For plants recovering from repotting or other major changes in growing conditions, fairly low levels (about 10 to 15% of the daily maximum) are often used, as for young seedlings.

The DLI is now the main concept used to quantify plants’ lighting requirements.

To avoid any confusion regarding the percentage of the daily maximum, daily light integral (DLI) is a much more representative parameter of plants’ energy requirements. DLI expresses the total flux of photons that fuels photosynthesis over an entire day. This measurement is just as valid when lighting is natural (varying depending on the time of day, as shown above) as when it is continuous (under artificial lighting), and even when illumination is obtained through a combination of natural and artificial lighting.

Very easy to understand

The unit of measurement may seem confusing, but it is actually very simple. The DLI is quantified by measuring the total amount of photons received per square meter over the course of an entire day.

Comme les photons sont de minuscules trains d’onde lumineuse, il y en a une quantité gigantesque reçue à la fin de la journée. On utilise donc le «mole» (6,02 x 10^²³) comme facteur d’échelle. Il s’agit en fait d’un simple facteur d’échelle, comme tous les autres facteurs multiplicatifs couramment utilisés, tels que le kilo (1 kg = 1 000 g) ou le méga (1 M bit = 1 000 000 bits). Le mole est toutefois un facteur multiplicatif beaucoup plus grand et vraiment difficile à imaginer. Les chimistes l’utilisent régulièrement pour paramétrer les réactions chimiques faisant intervenir une multitude de particules élémentaires, comme c’est le cas en botanique, bien évidemment!

The photon flux is an instantaneous measurement that allows the total DLI to be estimated.

Under artificial lighting, light intensity remains constant throughout the day. It is therefore possible to measure it at any time of day. The measurement then consists of estimating the total number of photons received on the collecting surface in one second, rather than over an entire day. Since there are 43,200 seconds in a twelve-hour day, simply multiply the photon flux per second by a factor of 43,200 to convert from one estimate to the other, as shown in the table above. Here, we will often use µmol (6.02 × 1017), a scale factor better suited to measuring instantaneous flux.

For those who would like to learn more, Mathieu Hodgson published an excellent article in December 2023 in which he presents a range of concepts and measurements specific to horticultural lighting. He also includes a table of recommended intensities for several indoor plants.

To summarize our series on artificial lighting for orchids

This article is the last in our series on artificial lighting. Here are the key points to remember:

1 – Lighting in our homes is often insufficient, especially in winter!

If you want to grow orchids indoors, you not only need to increase the amount of light, but also provide a full light spectrum to avoid chromatic deficiencies, both for photosynthesis and for the health of your collections.

2 – LED lighting offers the best lighting quality and is more efficient than all previous technologies.

Look for lights labeled “cool white” with a full spectrum, at 6,000°K or 6,500°K. The light produced should be focused on the growing space so as not to disperse energy that is so valuable for plant growth.

3 – Keep in mind that plants are small chemical processing plants and that light is their only source of energy.

To promote optimal growth and beautiful blooms, each plant must be provided with plenty of light. In fact, they must be given the maximum amount of light they can tolerate to allow each “mini-factory” to operate at full capacity. In practice, create a light environment that is as close as possible to the known requirements of the collection, then periodically observe each plant to ensure that the conditions provided are suitable.

4 – To take advantage of variations in light intensity, concentrate plants that require a lot of light toward the center of the shelves, where the intensity is highest.

This will be the ideal location for plants that are growing rapidly and preparing to flower, as photosynthesis will be maximized here. Plants with lower requirements should be placed on the sides, such as:

1. orchids that have finished flowering;
2. plants recovering from repotting or a weakening treatment;
3. young seedlings that cannot tolerate intense light;
4. any recently acquired plants that you want to gradually adapt to your growing environment.

5 – A few additional adjustments are recommended to ensure an optimal growth environment for our collection.

For orchids:

1. In particular, strict control of lighting duration must be provided for;
2. humidity must be maintained (especially in winter, when indoor air becomes much too dry);
3. there must be constant air circulation (24 hours a day);
4. the temperature must be lowered in the evening;
5. there must be very good quality darkness during the night. These adjustments are easy to implement, but they will be crucial to the success of our cultivation project.

6 – The light intensity recommendations found in most older reference books on orchids are 4 to 6 times higher than the plants’ actual needs – “under constant and continuous lighting.”

These recommendations are based on the maximum light level that the plant can tolerate in its natural environment at midday. In our homes, it is generally estimated that the actual artificial lighting requirements are between 15% and 25% of the maximum intensity measured at midday in a natural environment.

Acknowledgments

Thank you for following our series of articles on artificial lighting. Although the subject may seem a little daunting to some of our readers, it is of paramount importance for the growth and health of our orchid collections.