In Part 1 we discussed what the key forms of lights used in hydroponics were, and the equipment required to use them. This post will center on the role of Metal Halide and High Pressure Sodium bulbs.
By Sarah Pick
Metal Halide Lights
A metal halide bulb provides light by passing electricity through a gaseous blend of mercury and metal halides. It is a category of high-intensity discharge (HID) lamp and was developed in the 1960’s. The metal halides in the light bulb enhance the colour rendition of the light which is extremely helpful in cultivating plant life. You might see reference to the CRI (colour rendition index); this is a calculation of the colours given off by manufactured light in comparison with those seen in daylight.
Sunlight displays a variety of colours throughout the day and as a result supplies the different kinds of light required for plants to grow. But artificial light is man-made and different light models emit different colours. The scale operates from 0-100, with 100 being the best light at displaying the true colour of the object being lit. For example the bulbs used in our homes are normally tungsten incandescent lights and these emit a warm glow and have a CRI of about 95, whereas metal halide lights have a CRI of about 80 which is in the bluish frequency of light.
It is beneficial to know that metal halide lights produce blue light when working with hydroponics because blue light is what is needed for your seeds are to develop. The seedlings do well in this frequency of light as they mimic that produced at summer when outside plants are starting to grow. Many refer to metal halides to be the bulbs to use throughout the vegetative stage of growing plants. This is where the plants start to generate thick stems and large leaves which produce short solid plants. When the plants have developed suitably and are getting to the stage for flowering and forming fruit you need to change to high pressure sodium light.
High Pressure Sodium
A high pressure sodium bulb is again a type of high-intensity discharge light. Just like the metal halide lights high pressure sodium bulbs are a gas discharge lamp, but they consist of sodium as opposed to metal halides and emit a dark pink colour when initially turned on. This alters when the light has heated up to a pink/orange colour. These models of lamps have a CRI of 25 and consequently if used alone in hydroponics would produce tall, thin plants often referred to as “leggy”. As a result they are generally used in the flowering/fruiting phase of the plant cycle, because the red light triggers the reproductive phase. However it is worth noting, that as a result of the poor CRI the light will make the plants look pale and unhealthy but this is solely due to the colours of the light.
The two HID lights are usually employed in combination with one another with the metal halide used to initiate plant growth and the high pressure sodium to produce the fruit and vegetables. The only issue with employing these two lights is the expense of hardware. Each of these bulbs require different ballast. This can be expensive depending on how large your hydroponic garden is and the number of lights you need. That being said there are dual spectrum lights on the market that produce both forms of light and thus only involve the use of one ballast. This is probably the best bulb to use when starting out using hydroponics as the initial expense is limited
Take a look at Part 3 where we will talk about florescent lighting.